I'm deep in conversation with Spike Milligan. "Hello folks. Haaaaeeeello foooolks," he warbles tunefully, following up with a string of those garbled gurgles and nonsense noises that somehow convey a whole world of cheerful meaning. "What? What? What? We'll get the Irish guards to quell them!" he barks. Then he switches to Eccles - "Hello der!" - and has a little conversation with himself. "Eccles it's me, Bluebottle! Why don't you open the door?" "Okay den, how do you open a door?" "You turn the knob on your side!" "But I haven't got a knob on my side!"
For those readers who think we've landed in an episode of afterlife and are communing with the dead, don't panic. It's not, of course, the real Spike Milligan but rather his doppelgänger for today, the actor David Threlfall, who's taken a break from filming the fourth series of Shameless to give me his thoughts on life. He's taken to channelling Milligan - which he does with alarming ease and precision - because I've just pointed out that he looks eerily like the comedian. Oddly, though, when I'm confronted with him as his 53-year-old self - dyed blonde tufty hair, friendly if distant gaze, narrow features - his face doesn't seem to fit any of the characters I've seen him play at all. Either the camera plays tricks, or he's even more of a chameleon than he seems.
It's appropriate, though, that beneath those masks lies a close resemblance to another actor. Does he think he looks like Milligan? And would he like to play him? "Yes, it's been said," he admits, "it's interesting, that. I would love to play Milligan. I have very few heroes, but he was something special. The Goons were something I latched onto very early on, at school. Milligan's got a similar kind of outpouring, up-and-down character, I guess, as I have. Though I don't really have the excruciating problems he had, particularly with mental health issues."
His other childhood hero was another inspired lunatic - Vivian Stanshall of the Bonzo Dog Doo-Dah Band. "With another guy I went to school with, I took to writing versions of 'The Intro and the Outro' and putting it on the school noticeboard and watching kids and teachers come up to read it without at first realising what it was, that it wasn't a regular school notice, that it was a spoof." These reflections on his childhood interests lead him, with barely a pause for breath, to make a sudden tangential leap: "That was about the last time I did any writing." And then another, on to the subject of acting: "It just sort of pours out, it's instinctive and intuitive... I can't write books about acting, I can't get erudite about it, you either get it or you don't."
But if Threlfall can't - or doesn't want to - explain exactly how the acting process works for him, there isn't any question of the quality of the end result. Paul Abbott, the writer of Shameless, in which Threlfall plays the central role of wastrel father Frank Gallagher, thinks he is one of the top five actors in the country: "It is, I think, staggering, that Frank never gets nominated or wins Baftas. It's because David is too good. He doesn't just shine in one performance, he shines in everything he's done." And it is true that his talent has made its mark right from the start of his career. He made his acting debut as the rebellious but eloquent teenager Archer in Scum, the 1977 BBC Play For Today about borstal life by Roy Minton and Alan Clarke. But the controversial play wasn't broadcast until after Clarke's death in 1991, and Threlfall didn't reprise his role in the 1979 film version. Instead, the RSC brought him to widespread public attention by casting him as the fey simpleton, Smike, in its marathon 10-hour adaptation of Dickens's The Life and Adventures of Nicholas Nickleby (televised for Channel 4 in 1982).
I get the impression that Threlfall sees Smike as something of a two-edged sword: it made him famous, but it took him years to put the part behind him. That's perhaps because it was a showy role, I suggest. "It didn't feel showy," he says. "It was very internal. I physically adjusted my system in a way that was difficult, and it took me a while to recover from it. Doing eight hours in a show, there was no room for me, there was no room for David. It wasn't easy for me or others in the cast. But then, you know what they say, no pain, no gain." What was hard about it exactly? "I think I want to keep that to myself, because if I name what it was..." He trails off and there's a pause. "I just turn up and do the work," he says. Again, that reluctance to name things, perhaps because he sees it as private, perhaps because he doesn't like labels: he has castigated journalists for calling Shameless a comedy drama, insisting that it is a drama only, and says to me, "That's the nature of writing, you compartmentalise things."
This holding back - the sense that he's putting up a shield, refusing to show his hand, "doing enigmatic" for you - also comes across in some of his performances, notably a gaggle of repressed, upper-class misfits who include politician Leslie Titmuss in John Mortimer's Paradise Postponed (1986) and Titmuss Regained (1991), and the father-and-son duo of Prince Charles (in 1993's Diana: Her True Story) and Prince Philip (2005's The Queen's Sister).
Though the starring role in The Queen's Sister was that of Margaret, played by Lucy Cohu, Threlfall still stood out as Philip, who came across as something of the family thug. "It was only about four days' work and it made a real mark," Threlfall says, sounding surprised. "Though the director, Simon Cellan Jones, should get a lot of credit for that. He told me, 'It's too much of an impersonation, you need to come back'. Because I'd played Charles before, I just aged him up, as it were. It wasn't meant to be an impersonation, but once I'd got it in my head it was difficult to get the impersonation out," he admits. Then he adds, going off on a tangent about acting again in a way that's starting to seem typical, "I just like getting up and pretending. It's the game of pretending you're other people, that's the liberating thing."
Ever versatile, Threlfall can do both working-class as well as upper-class repression if the role calls for it, which it did in last month's wartime drama Housewife, 49, written by and starring Victoria Wood, when he played Wood's cold and seemingly unresponsive husband. He can do comedy: he starred with Robert Lindsay and James Ellis in Nightingales (1990), a surreal and unconventional number about nightwatchmen in which Threlfall sported truly bad hair as the thuggish Bell. He's played madmen (Edgar in King Lear in 1983), Nazis (Dr Wilhelm Kritzinger in 2001's Conspiracy) and Roman Emperors (Constantine in last year's The Battle for Rome). Towering over all of them, though, is the role he's made his own over the past three years or so - Frank Gallagher, the feckless waster who spends most of his energy trying to wangle money for drink and drugs, which he'll happily steal from any one of his six children if he has to. Some of Threlfall's more bottled-up characters clearly need to let the lid off a little: Frank needs to put it back on. But despite his flaws, he has the same scumbag vitality that his whole good-hearted low-life family has, the very quality that makes Shameless such an enjoyable show.
Did Threlfall know from an early age that he wanted to be an actor? No, is the answer. "I wanted to be a footballer." He came from a small, working-class Manchester family: his father was a builder and he had one brother. The signs were there, though, that he had the makings of something different. "I was a bit of an odd child - my dad was thought of as an eccentric, so there's some genetic thing there... I didn't belong to a gang or anything at school, I had a good friend for a while, and I used to redo Morecambe and Wise gags to try to make him laugh. Quietly. We were a syndicate of two." Later on he did a stint at youth theatre that he says he really enjoyed: the first play he ever starred in was The Rising Generation, a 1960s experimental drama. "Some of the fourth formers would insist on spitting on us;" Threlfall recalls. "I suppose it was a good grounding for what came later." The second play he did was Arthur Miller's The Crucible, in which he played the lead role of John Proctor. "That was tremendous," he says, repeating "it was an emotive experience" a couple of times, thoughtfully. "It was so emotive, I thought 'Wouldn't it be great if you could do that for a living?' But I just didn't make the connection at all as to how you could do that, so I went to art college in Sheffield." Art college didn't make him happy, so he threw it in and did labouring for a year while he "thought about what to do... and then decided I would try drama school". And that was it.
Does Threlfall think the roles he plays have had common characteristics? "That would imply a choice, which I don't think I've always had," he says. "Though when I did Someone Who'll Watch over Me" - a play about hostages written by Frank McGuinness and staged in London in 2005 with Jonny Lee Miller and Aiden Gillen - "the director Dominic Dromgoole said to me, 'You're very good at looking at the English and playing any sort of social scale within that.' I'm sure there's a palette that emerges but the only thing I like to do is something different from what I did last week." And indeed, he hasn't always had a choice: towards the end of the 1990s there were about two years during which he struggled to find work at all and was on the brink of becoming a motorcycle dispatch rider when he was finally offered more theatre. "When you're out of work, it's a very humbling experience," he has said.
So why does Threlfall think Shameless is such a successful show? "It's really only dawned on me the last year that it is," he says, before getting cagey again. "I just do the work... You'll have to ask someone else that, it's a question for the audience. I just do what I do." In fact, he doesn't really watch Shameless, or indeed much TV at all, he says, adding that he prefers to spend time with his family. Which, like the one he grew up in and unlike the Gallaghers, is small. He's married to the actress Brana Bajic, who appeared as Lena in the second series of Shameless, and has two children: "Boys. Eleven and seven. No, in answer to your next question." What was my next question? "Do they watch the show?" I hadn't been going to ask that, but never mind.
I say that a friend of mine, struggling to find words to describe Frank, called him both a "philosopher poet" and an "idiot savant". Threlfall considers the question. "Philosopher poet? Yes, I'd go with that. Idiot savant, yeah..." he adds, sounding less convinced. "I think Frank is an intelligent man. In fact, I'd say we were equally as intelligent as each other. But he took up drinking as a mission at 24, 25 and just stayed on that track... though the one thing I did say to the directors was that Frank shouldn't always be drunk. He should be surprising."
He won't reveal what might be happening with Frank in the fourth series, saying, "I've no idea, I wouldn't bank on anything happening, I wouldn't put any money on what this show is trying to depict. I don't know what's in store for Frank next week, let alone next year." This turns out to be untrue, unless he's misunderstood what I'm asking. The fourth series has nearly wrapped when we speak. Abbott, unlike Threlfall, is willing to give me the lowdown: "Frank's ex-wife comes back and he's trapped between two lesbians, neither of whom he really wants but one of them's got money so he needs to pimp the other one." But this storyline doesn't mean that Frank grows up at all, Abbott says. "Frank is a 12-year-old boy with a nine year old's mentality and a 50-year-old's vocabulary. Emotionally you can't really extend him beyond where he is now."
Abbott admires Threlfall, whom he also describes as a friend ("there aren't many actors who I see socially, perhaps only three or four - you meet a lot that are nice people, but they just don't chime on the same wavelength as mates"). He says that Threlfall, who was only cast as Frank at the last minute after the previous actor booked for the role was rejected as too young, is "one of the most dextrous people I've ever met. I write stuff for Frank where I think that I'm the only person who really knows what it means. Threlfall dives onto it right away, he's so onto the rhythm of it, you watch it coming out and think, 'Oh my God, that's exactly what I wrote!'."
Having said that, Threlfall is not the easiest actor in the world to work with, Abbott admits. "David is needy. He's very good company, very irreverent, but at the same time extremely fragile, which is why he can pull all of those characters out. He can need to talk things into place, and you think, 'Oh God, all this actor's motivation stuff - just read the lines!' Actually, though, I've never said that to an actor because when they ask questions you know what they're doing, they're calibrating, and David calibrates so finely that there's no one quite like him. So it can be tricky if he doesn't quite get something. He won't let it go and he'll want to keep talking. He doesn't pester you but if you haven't answered his question there'll be another phone call the next week."
I say to Abbott that when I saw Threlfall as himself he didn't look like any of the characters I'd seen him play. That's the thing with actors, he says. "Once they get the shoes and the wig on the whole psychology adjusts. Which is quite psychotic - it's an occupational psychosis, where they transport themselves into someone else's body... I'm not sure I trust them... [though] I trust Threlfall with stuff, he's so trustable."
It's striking, the contrast between Abbott, a writer and therefore a man you might expect to be shy or withdrawn, who's very open, even blunt, with his thoughts; and Threlfall, who - though he's friendly in person, and God knows uninhibited enough as an actor - seems to prefer to hold back. The hellish dissipation of Frank Gallagher notwithstanding, there's something fastidious in his manner. It's all part of his talent, though, I think: his dislike of easy labels, the freewheeling way he talks, the guard he puts over his private thoughts, because what he clearly likes to do is suspend judgements while he fine-tunes to a precise degree, like a musician listening for the right note. I do hope one day he gets to play Spike Milligan, in that drama about the comedian's life which is surely coming. Someone will. And I suspect he could manage that particular bout of occupational psychosis better than most.
The fourth series of 'Shameless' begins on Tuesday (10pm Channel 4), and is previewed on page 31