"These go to 11." This celebrated phrase from This Is Spinal Tap, Christopher Guest's deathless, semi-improvised 1984 "rocku-mocku-mentary" has become part of our language. It comes from the scene where Spinal Tap's ludicrously self-deluded guitarist Nigel Tufnel (played by Guest) shows off that his amps are noisier than any other rockers' – because the dials all "go to 11".
Fans of the movie, known as "Tappists", still constantly use the phrase – a fact that leaves Guest proud, yet bemused. In a break between filming scenes of his latest offering, Family Tree, he says that, "Fans come up to me all the time quoting lines from This Is Spinal Tap.
"It's odd to think that a film that was made so long ago has stuck with people. I've heard that the Oxford English Dictionary now includes the phrase 'go to 11'. If nothing else, we've achieved that!"
So it is quite a coup for the BBC to have secured the services of this celebrated film-maker – also responsible for such memorable semi-improvised cult movies as Best in Show, A Mighty Wind and Waiting for Guffman – for his first TV project.
In Family Tree, a new eight-part series co-produced by HBO, which begins this week on BBC2, Tom (played by Chris O'Dowd from The IT Crowd) is at a loss after being sacked from his job and ditched by his girlfriend. He is left a mysterious box of possessions by a great aunt he never knew, which prompts him to join forces with his best friend, Pete (Tom Bennett), and his sister, Bea (Nina Conti), in tracing his family tree. And so begins a belated quest for a sense of identity.
O'Dowd, 33, is on a break in a south London park. He has been filming a scene about a distant relative taking part in the 1948 London Olympic Games. We are surrounded by a squad of extras wandering around in vintage "Austerity Olympics" Team GB vests and rigorously Brylcreemed hair.
The actor, who read politics and sociology at University College, Dublin, admits that when he was offered the part of Tom, "I could scarcely believe I was going to be working with Christopher Guest. In our student digs, we had Spinal Tap on all the time. It ran on a loop for three years while I was failing my degree.
"I love lines like, 'You can't dust for vomit.' But I don't have a favourite line – just give me the whole script! So it's great that Christopher thought I could play this role; although it's obviously a big mistake on his behalf!"
In fact, the improvisation keeps the dialogue in Family Tree really fresh and often surprising. For instance, when Tom is driving with his sister to see his semi- estranged father (played by Michael McKean, aka David St Hubbins from This Is Spinal Tap) and his dislikeable Eastern European stepmother, Tom speculates, "Do you think we'll go round and he'll say, 'I murdered her. She's in the bath, just stewing in her own cabbage'?"
Access unlimited streaming of movies and TV shows with Amazon Prime Video Sign up now for a 30-day free trialSign up
The 65-year-old British-American director explains that, "When the actors are given a strict back history and they know where their character went to school, what music they like and who their friends are, it allows a spontaneity that doesn't come with other forms of comedy.
"I've done scripted stuff before, but my choice would always be to work in this way because it elicits what I need. This is the most fun way to work, and one of my chief aims – believe it or not – is to have fun. Once you get the people you want, it's like playing music."
The actors confess that initially they found the improvisation process pretty scary. Bennett admits that, "On a normal drama, if it's terrible you can say, 'It's the script's fault. I was brilliant'. But with this, I can't blame the script because I have just made it up on the spot! But it's been amazing fun. I might as well retire now – this could be the peak of my career!"
So what drives Tom's search for his ancestors? "Boredom," says O'Dowd, who has also had starring roles in such movies as Bridesmaids and The Sapphires and a recurring part in HBO's Girls. "He's very lost. That's why he starts tracing his family tree. He has nothing to do during the day, and Jeremy Kyle is not his thing. I don't get that. Who doesn't like Jeremy Kyle? He's the Piers Morgan of daytime TV. He's that good!
"Tom starts learning more about himself and the people in his background. He learns that some things in this family are cyclical. For instance, he discovers that, like him, his great-grandfather was cuckolded and finds solace in that. His father is not very sympathetic to his plight, so finding empathy in the family tree is a pleasant experience for Tom."
O'Dowd, who hails from County Roscommon in Ireland, goes on to reveal that, in a strange echo of the sitcom, he has recently been examining his own family's roots. "Weirdly, I had started to look into my family history in the last two years because I wanted to buy a castle. It was a basic two-up, two-down Irish castle with no roof – it was a wreck. Irish castles aren't that fancy. The only reason I was interested in it was because it was called O'Dowd Castle. But in the end I didn't buy it."
The actor, who married TV presenter Dawn O'Porter last year, says that he made some surprising discoveries about his family during his research. "I found out that my great-grandfather was a bigamist. He was a travelling salesman and had a second family in Birmingham. Apparently that strand of the family gave birth to a George O'Dowd. I've mentioned it to Boy George, and he's quite into the idea that we might be cousins!"
In Family Tree, O'Dowd has particularly relished the bizarre things that Guest has asked him to do as Tom. For example, he says, "Tom discovers that his great-grandfather was the back of a pantomime horse. I don't want to give too much of the story away, but there is a falling out between the back and the front of the horse…
"What's it like being the back of a pantomime horse? What's interesting is that there are 50 years of history and 50 years of aromas inside that costume. In the show, we flash back to a famous race for costume animals at Sandown Park. It was quite surreal playing that. You think, 'I'm a grown-up. I have a wife and a mortgage, and I nearly bought O'Dowd Castle, and I'm in a costume horse racing a panda at Sandown Park. This is an amazing job!'"
The director is excited about a potential second series of Family Tree. "There is no limit to the idea of Tom tracking down people in far-flung places. I like to eat good food, so obviously Tom would discover ancestors in parts of Provence and Tuscany!"
Guest, who has been married to the ctress Jamie Lee Curtis since 1984, is completely tuned in to British humour. He feels right at home in the UK. After the death of his British father in 1996, the director became The 5th Baron Haden-Guest of Great Saling.
He says that, "Apart from New York, London is the place I feel closest to. I've had two passports since I was a child. I was one of the hereditary peers. But in 1999, we were handed a note saying, 'Get out. Set one foot in here again, and we'll kick your bum!'
"I still have a robe and a title, but I don't go to the House of Lords now. I sat there for several years, but you are only allowed to vote once you have made your maiden speech. My maiden speech was just a load of jokes, so I decided not to do it. I thought it would be disrespectful."
Guest contemplates the question he is most often asked: would you ever consider making a sequel to Spinal Tap? "We have been asked that so many times, but we said what we needed to say in that film. We all agreed that would be that."
He feels equally ambivalent about another Spinal Tap live tour. "The problem is that it takes eight months to organise, and we're old people now. But it's really fun to do. It's very surreal and has gotten more so over the years.
"We did an acoustic tour four years ago when we played Glastonbury and Wembley. I'm a grown-up and I'm standing in front of 130,000 people at Glastonbury wearing a wig and playing loud music – it's almost an out-of-body experience. It's hard to know how it happened, but we really appreciate it. It's the fantasy of all time. We've been handed a card which says, 'You get to do it, and then you get not to do it'. That's the biggest gift."
As for O'Dowd, he says he is looking forward to the prospect of himself and Richard Ayoade playing the eternally single geeks Roy and Moss in the final ever episode of Channel 4's The IT Crowd. "I imagine the opening scene would be Moss leaving the office and saying to Roy, 'Anyway, I'd better get back to my wife and child now. Only joking!'"
After that, O'Dowd, who will also be seen later this year in the mega-budget Thor sequel, will no doubt be going back to Hollywood. After all, he is such a big star now that we often see his face plastered on the side of buses. How does he feel about that?
"I put most of those posters up myself," the actor deadpans. "That's why I have been working so hard. The bus advertising side of my career is going very well. I'm nailing the transport system right now!"
With improvisation like that, Guest's comedy is clearly in safe hands.
'Family Tree' begins on Tuesday 16 July at 10pm on BBC2
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies