James Corden is feeling nostalgic. He's back at the National Theatre for the first time since appearing in The History Boys, the 2004 Alan Bennett play which swept the world – transferring to Hong Kong, Wellington, Sydney, Broadway, the West End and the silver screen – and made stars of its schoolboy heroes. "I feel a bit like when we came through the doors on the first day of rehearsals of that play, from that minute on, my whole life changed," says the 32-year-old actor, staring out, mystified, at the Thames. "No one could have predicted on day one of rehearsals, that a year and a half later we would have shot a film and all be living in New York. It was surreal."
Nor could anyone have predicted that Corden, the chubby class clown, would go on to graduate – some 500 performances later – with top marks, winning a mantelpiece of awards for Gavin and Stacey, roles in assorted British and Hollywood films, and his own sketch show. Further setting himself apart from his former classmates (Dominic Cooper and Russell Tovey, to name but two of the over-achieving young cast), Corden has carved out a lucrative, everyman television personality too. A sort of Smithy-for-hire with his own sports quiz on Sky, he has hosted the Brit Awards twice (once with Kylie), hung out with the England squad – fishing with Terry, golf with Gerrard – for his own live World Cup panel show, recorded a Number 1 football anthem with Dizzee Rascal, and taken over from Ricky Gervais as Comic Relief's favourite go-to celebrity piss-taker.
None of this, though, would have happened without The History Boys and the quiet encouragement of Bennett, who suggested that the cast's resident big mouth might think about writing some of his rehearsal-room banter down. "It formed a lot of my writing muscles," says Corden. "When someone like Alan says it, it just gives you a bit of confidence. He's incredibly supportive but he would have been as supportive if I'd said, 'I want to give this up and do fly-fishing.' The whole experience of being in the play gave all eight of us confidence, really."
James Corden? In need of a confidence boost? Surely not. "No, no, I don't think I've ever been lacking in confidence," he admits. "There's a difference between wanting to appear confident and actually feeling confident. I think there have been many times when I've over-compensated for how nervous or out of place I feel. I was like that at school."
He's hidden it well so far. The parts he's played tend towards the loud and rambunctious, with flashes of heart-on-your-sleeve sincerity – most famously, the hilarious motormouth/ unashamed softie Smithy in Gavin and Stacey – while his comedy outings frequently end with flashes of belly flab. Off-stage, there have plenty of scene-stealers, too. For a while, a couple of years ago, an awards show wasn't an awards show unless Corden had turned up, snogged someone on stage – David Walliams, Daniel Radcliffe, whoever – shocked a giggling audience or had a set-to. There was the time he presented the Empire Best Actress award to an absent Keira Knightley and blamed her no-show on the fact that he'd "been shagging her solid for the last three days", while last year he got embroiled in an ugly spat at the Glamour Awards when Patrick Stewart, unprovoked, called him fat. Corden retorted by calling him "an old man" – and a taxi.
"There's a fine line between being confident and appearing to be arrogant. I would hate to think that I've ever been arrogant but I can absolutely believe that I may have appeared to be so at times," he says, typically unapologetic. "That just comes from where your head is at the time." And that could be any number of places. His co-writer on Gavin and Stacey and best friend Ruth Jones has described him, politely, as "very rollercoaster-y" – or, as Corden puts it, "like a bipolar child".
Certainly, he's sending out mixed messages today. He claims that he's jittery about this interview – "I get more nervous than you would ever know" – but minutes earlier, he's bowled backstage and before he's sat down, is taking me to task over a piece that I wrote three years ago about the fate of the History Boys. "You said I'd had 'mixed success' since the play. What was that about?" I never wrote that, I say. "Well, somebody said that. Ridiculous!" It's the same gauche braggadocio, a kind of honesty Tourette's perhaps, which led the actor to berate the throng at the 2008 Baftas for only giving him two awards, not three. It's as if he can't quite believe his own success.
Otherwise, Corden is in reflective mood today and, just so we're clear, is not at all like Smithy. Softly spoken and self-consciously polite, he even offers me half of his crayfish sandwich. The only thing he shares with his brash Billericay alter ego is a propensity for alarming bursts of sincerity. He's not very funny; in fact, he's rather serious. Has the History Boy grown up? "Yeah. Mostly in the last year and a half, I would say."
He's back at the National to star in Richard Bean's One Man, Two Guvnors, which updates Goldoni's 1743 Venetian comedy to 1960s Brighton. Corden plays the anti-hero, a skint skiffle-band reject, wheeling and dealing as he tries to keep his various bosses and lovers apart. The play reunites him with Nicholas Hytner, who directed him in The History Boys. The director has said that he cast the 26-year old "seven and a half seconds" after he walked in to audition. This time, he called him up and offered him the lead over the phone. Corden didn't even have to ask which play it was. "I don't care, really. I genuinely believe that if you're an actor in this country and you get to work in this building then you're one of the luckiest in the country." Today he's nursing several farce-related injuries, including a scratched eyeball – "I fell flat on my face. Actually on my face." – and a smashed-up knee. There's no doubting his commitment. "You just don't want to let Nick down. That's all. I couldn't bear the thought of ever letting him down."
After the National, he's sitting down to write his new show, The Wrong MANs with Mathew Baynton (aka Smithy's gormless friend Deano). A thriller action comedy "which couldn't be more different from Gavin and Stacey", it has just been commissioned by BBC2 to screen in autumn 2012. "I'm at my happiest writing things and being in them," he says. And very good he is at it, too. Would he ever write a play? "I've started a couple but you've got to earn your stripes. I would love to one day but I think perhaps I need a touch more life experience."
He's working on that. Seven weeks ago, Corden became a father. "It's just an overwhelming rush of love, a feeling of, 'I would do anything for you and I met you 20 seconds ago'," he says, showing me a picture of baby Max on his phone. "It's quite strange when you have a baby, being a man. He doesn't really need me. He needs Julia, he needs food and that's it. It's just about support, really and it's been difficult." Julia is Julia Carey, a former PR for the Save the Children and Corden's fiancée. The pair were introduced by Dominic Cooper, Corden's ex-housemate (who still lives five doors down from them in Primrose Hill) last year. He proposed on Christmas Day and they are getting married as soon as they can get round to it. "Both of us know we should be talking about it but we're both so tired, we don't."
These days, he says, his social life consists of going home, helping out, learning lines and trying to sleep. "And I couldn't wish to be happier, is the truth. Both personally and professionally. That's probably the first time I could say that. There have been times when professionally I've been incredibly happy but personally I've been more lost than ever. Likewise, before that personally I'd always been very happy but professionally I'd felt somewhat creatively unfulfilled."
James Kimberley Corden was born in Hillingdon, and grew up in Buckinghamshire in a "little cocoon of love", the middle child between two sisters. His mother is a social worker and his father was a musician in the RAF and now sells Christian books. As a teenager, Corden played cornet in the Salvation Army band. "Out of a congregation of 60 people you could count the number of Christians on one hand. It was just full of frauds. My parents are the ideal Christians – very free-thinking, liberal and open." Is he still religious? "I guess I have a faith. I have an overriding feeling that all of this can't be for nothing. But then I also fully understand that it might be."
At school, Corden was the boy most likely to perform, attending the same unusually fertile after-school drama club in High Wycombe as Eddie Redmayne and Aaron Johnson. He left with two GCSEs and set off for the stage. "I certainly didn't have a fallback plan." Aged 17, he joined the chorus of the West End musical Martin Guerre but soon grew bored of standing at the back. Not long afterwards, he was sacked from a film adaptation of Martin Amis' Dead Babies for bad attitude.
Still, his talent outweighed any quibbles and a fruitful period of fat-boy roles in Shane Meadows' Twenty Four Seven and Mike Leigh's All or Nothing via a Tango advert, Fat Friends and Hollyoaks (which he hated) followed. It was on Fat Friends that he met Ruth Jones. The rest is television history – how Corden went to a wedding in Barry and the pair started talking about a series "where nothing really happens and yet within that everything happens". Deceptively simple, Gavin and Stacey's beauty lay in its warm heart, a million brilliantly observed eccentric details, and a dream cast with Alison Steadman, Rob Brydon and Julia Davis among the supporting players. The first episode was watched by 500,000 people on BBC3. By the final episode of the third series, which aired on BBC1 on New Year's Day 2010, 10.3million were tuning in for Nessa's will-she, won't-she wedding (and, brilliantly, a cameo from John Prescott. "I liked him actually", says Corden. "He was terrific.").
There won't be a film but Corden wouldn't rule out another special and would love to write again with Jones. "We'd both be disappointed if we didn't at least try. We talk all the time." He recently asked her to be Max's godmother "It's very much a friendship first and a working relationship after that. I love her dearly. The thought of being in a room with her again, whether that's to write Gavin and Stacey or something new, is sometimes all that spurs me on."
Twelve years older than Corden, Jones also proved a "tremendous support" when the wheels, inevitably, started to come off. Having spent almost a decade working for his big break, everything happened at once for the young actor. He returned from Broadway, filmed the first series of Gavin and Stacey, and broke up with his girlfriend of nine years. "So I was single for the first time in my adult life, coupled with being a little bit famous. It's a bad mix, that. Not a nice, healthy concoction. I think that played a part in my behaviour at times."
As the awards started to come in, Corden got a little carried away. There was another break-up (this time from Sheridan Smith), very public wooings of Lily Allen (telling her on her chat show, "I don't think you realise how lovely you are") and the X Factor winner Alexandra Burke (sending her a £4,000 watch), and countless 3am benders at the Groucho. Did he go off the rails? "Listen, I wasn't like Pete Doherty. But I felt completely lost for a very long time. I don't know what dreams are when they come true – they just sort of vanish. What you need, when suddenly it seems like all your dreams come true, are those long relationships. You anchor yourself to them and you can see it with perspective. But when you don't really know where you are because you're sleeping on your mate's sofa, it just starts to pollute you a bit. That's what happened to me."
In true Uncle Bryn style, Brydon took him to one side and told him he was "in danger of becoming a twat". The greater threat, though, was over-exposure. For a while in 2009, Corden was everywhere – accompanied by his Gavin and Stacey sidekick Mat Horne. Having hosted the Brits together they unveiled their own sketch show, Horne and Corden. Billed as the Noughties answer to Morecambe and Wise it was, given the talents of those involved, a dismal disappointment, a series of puerile skits about gay war reporters and Corden's wobbly paunch. The critics had a field day. "We sat down with an idea to make a show that would appeal to us when we were at school. So it shouldn't have come as a shock that it was reviewed badly. It would be like sending a 15-year-old to review The King's Speech."
Fuel was added to the fire when, two weeks later, Lesbian Vampire Killers was released, a schlocky horror that, apparently, not even a father could love. "I said to my Dad, 'what did you think?' And he said, 'Well, it's not All or Nothing, is it?'" Did you know it was that bad? "I don't think I even thought about it. When I was offered the lead, there wasn't a plethora of offers coming in. You just say, 'Wow! Really? Me? Ok!'", he sighs. "I was in a film which wasn't very good. A lot of people are in films that aren't very good. In fact, most films aren't very good".
It wasn't quantity that was the problem, he says, but quality. "There are a lot of people who are on television a lot more than me. But not one of those things was very good. That's the point."
As for Horne and Corden, they went from spending 230 days of the year working together to barely talking. "It was quite a bruising time. It was important that we had some time apart. I think we probably both got a bit swept up in all of that." Would he work with him again? "Of course. He's wonderful. It just has to be the right thing. The absolute truth is that neither of those things was good enough. You can't do anything without making mistakes otherwise you just sit on your own, staring at rugs." He has, though, learned from his mistakes. "You realise that it's more important to do four good lines with good people in a good film than have 400 lines in a bad film."
He's recently tried out Hollywood. Having appeared in Gulliver's Travels ("I was the guy stood behind Billy Connolly, which was fine by me"), he spent last winter shooting The Three Musketeers in Bavaria with Orlando Bloom and Milla Jovovich. "Working on something of that scale was tremendously exciting but when it comes to it, you're acting for about 12 minutes a day. Most of the time you're just waiting for things to happen." He likes LA – "because it's sunny all the time. But it's not the postcode, it's the work that you get to do. I would rather work here with Nick Hytner or Mike Leigh or Shane Meadows or Rufus Norris than be in Alvin and the Chipmunks: the Squeakquel."
On the whole, Corden is not the coulda-woulda-shoulda type. "I would much rather regret doing something than not doing something," he agrees. Does he love being famous as much as he appears to? "Yeah," he grins. "I do. It's a picnic every day. A joy. It's a succession of people coming up, saying really lovely things and just wanting a photo. The parameters of what's involved have never been clearer. If you write your own TV show and star in it, or you go on Jonathan Ross, you know exactly what you're getting in to."
And he wouldn't change that for the world. "Do you know what happens on the day of the Brits? You have a dress rehearsal in the afternoon where the Arcade Fire basically play for you and you alone. The 02 was empty and I was stood 15ft away, watching Arcade Fire and then Tinie Tempah and then Mumford and Sons..." His eyes light up. "Now tell me that's not the greatest job in the world."
'One Man, Two Guvnors', National Theatre, London SE1 (020 7452 3000) 17 May to 26 July in rep; touring 27 September to 29 October ( www.nationaltheatre. org.uk)