Heaven knows what the 18th-century writer Laurence Sterne would have made of it, as it has precious little to do with his novel Tristram Shandy. But the show-stopping moment in Michael Winterbottom's A Cock and Bull Story - an "adaptation" of Sterne's book, very loosely speaking - is an extraordinary spate of ad libbing by actor Rob Brydon on the precise yellow of his teeth: "Barley Meadow... Tuscan Sunset... Pub Ceiling... You could decorate a child's nursery in this colour."
This is far from Brydon's only prize moment in the film: others include an eerily accurate impersonation of Roger Moore, and several less exact stabs at Al Pacino. Brydon's joyously non sequitur-filled performance brings a distinctive spice to Winterbottom's film-about-a-film, which is a rambling, crazy-paving contemplation of Sterne's notoriously unfilmable and uncontainable comic novel.
One of the film's appropriately self-reflexive ploys is to have its lead actors Steve Coogan and Rob Brydon play themselves (or at least, actors called "Steve Coogan" and "Rob Brydon") who endlessly tilt at each other in a rivalrous double act. An established TV comedy fixture for a little over five years, the Swansea-born actor may not be as universally recognised as Ricky Gervais, or the Little Britain duo, for whom Brydon has been both script editor and guest star (as the towel-wrapped husband of Bubbles de Vere). But he has a devoted contingent of fans, is very closely watched by critics, and is proving one of the least predictable players in current comedy, especially now that he's branching out into drama and the big screen. He first appeared on BBC2 in 2000, seemingly out of nowhere, as the hapless divorcé Keith Barret, cheerfully maintaining his dented shield of blithe denial in the one-man tour de force of minor-key comic bleakness, Marion & Geoff.
Then he played a sextet of semi-realist British grotesques - some of whom made Keith look a natural alpha male - in Human Remains, his collaboration with Julia Davis, prior to her stygian Nighty Night. He also created Peter De Lane, a pompous TV hack meticulously talking us through episodes of Bonanza and Mr & Mrs in his ITV series Director's Commentary.
But Brydon, 40, has recently turned away from the comedy of emotional carnage for more benign material, playing a gauche astronomer in Australia in the tepidly good-humoured series Supernova. He even gave Keith a new lease of bushy-tailed life in a kinder, tamer variation on the overmined mock-talkshow format, The Keith Barret Show, quizzing B-list showbiz couples about their marriages - a one-joke idea that Brydon has nevertheless spun out into a successful fixture on the stand-up touring circuit.
It's routine to wonder, when interviewing protean comics, just who you're going to get when you walk into the room, if only because these shapeshifters often lay on a performance once the tape is running; they do it either deliberately to keep their distance, or as an involuntary tic, or because they feel obliged to provide good celeb copy. I can't help wondering whether Brydon in any way resembles the affably distracted "Rob Brydon" of A Cock and Bull Story, who just happens to share his face, name and CV. The first person I see when I enter the room, in fact, is Brydon the tense suburbanite, pacing around with an anxious expression, finishing a call on his mobile. "If you'd come in a moment earlier," Brydon says, his Welsh accent just a shade less pronounced than Keith's, "you'd have heard me yelling at the garage" - although he says it so quietly you suspect he really means some gentle tut-tutting imprecation.
Sitting down, Brydon tells me he's about to move house, from Richmond to another location nearby. Do I know the area? Vaguely, I reply - and sure enough, he's performing, launching into a note-perfect Cook and Moore imitation. "'Do you remember my Uncle So-and-so?' 'Vaguely.' 'That's how people remember him, Dudley. Vaguely.'" A Cock and Bull Story is a remarkable film, I start to say... and he's off on another riff: "Remarkable in a good way? Just so we know where we are. I sit here all friendly and nice and then halfway through, realise, 'Oh, he meant remarkably bad.'" Remarkable in a good way, I reassure him, and he seems to relax. I ask about the film's seemingly ad hoc duelling between him and Coogan. A Cock and Bull Story is not quite as improvised as it seems, Brydon claims. Much of it is, he says, but it's not always easy to tell. "A lot of it that just seems like he left the camera on is actually scripted."
Michael Winterbottom cast Brydon in a small role in 24 Hour Party People (2002), and then approached him to play Uncle Toby in what was originally planned as a TV Shandy in 10-minute episodes. Brydon recalls, "I went to Waterstone's in Charing Cross Road, saw the book and ran screaming away from it." At the read-through, he tried doing Toby as a posh twit, along the lines of his Peter De Lane. He puts on a breathy Chelsea Arts Club voice: "Like Basil Brush. It got big laughs round the table, and Michael said, 'Oh no, I don't think you should do it like that.' That's the only time that's ever happened to me." He sounds genuinely aggrieved.
One of the film's most engaging strands is the way that "Rob Brydon" consistently upstages the supposed lead Coogan, who plays both Tristram and his father Walter; Brydon is a constant thorn in his side, Costello to Coogan's Abbott, or indeed Basil Brush to his Mr Derek. "In the original script, the Rob character was far more deferential to the Steve character. Michael's idea was that that relationship would mirror Toby and Walter's relationship. I wasn't that happy with it," Brydon says. His angular Joker face, with its shovel chin, lengthens an anxious notch further. "There are some things that I don't mind projecting, even if they're not true, but I didn't want to project that, that I was kind of puppyish around him."
It's inevitable that Brydon is perceived to some degree as his co-star's protégé: it was Julia Davis passing Coogan a tape of Brydon that led to Marion & Geoff being produced by Coogan's Baby Cow Productions. "There's a perception that he's kind of a mentor figure," says Brydon, "which is fine. But you only want to take that so far, otherwise you're seen as an appendage or something. So I was a little uncomfortable with that, and there were scenes where I'd be asking his advice. I mean, I may ask his advice occasionally... but not in the way it was written." Their relationship is "playfully undermining," Brydon says, "quite competitive". But he seems to be upstaging Coogan quite ruthlessly. "I am, I am upstaging him in the actual film, I really am," Brydon nods animatedly. "But we're cool about that, because it's all part of the game."
When Brydon talks about improvising, it's apparent that he takes technique very seriously, that for him it's not insouciant, off-the-cuff fun. I mention the business of the yellow teeth. "We were just sitting there, and you think, 'I want to be different - what can I do that's different?' I really thought, 'Here's a chance to put a mark on this'. And I was aware as I was doing that" - he curls back his lips in a dentist's-chair grimace - "that that's quite an interesting image, that's quite different, you don't see that very often. If I hold it, it's inherently funny, somebody putting themselves in a very weak position by going..." - and he bares his incisors again.
Unless it's all feigned, Brydon exudes a bristling anxiety that may be partly fear of failure, partly Keith-style eagerness to please. Asked how his Roger Moore impression got into the film, he launches into a complicated story about shooting the film's electronic press kit. "I thought, 'Right, I'll be good at this - I'm going to improvise some stuff, this is what I do, this is my territory, this is a chance for me to be good in this film...' And then I was bothered by the fact that Michael wasn't getting any close-ups of me."
But Brydon doesn't try to hide his delight in success, in catching everyone's attention, as he does with his impersonations. "There's a bit in the Pacino bit," he grins with transparent relish, "where the voice is just right and you see audiences go - [in a blissful, high-pitched laugh] 'Oh hooo hoo hoo!' It just hits a sweet spot, like with a tennis racket." Given that he waited years for a chance to be noticed, it's understandable that Brydon is making the most of it now. After studying at the Royal Welsh College of Music and Drama in Cardiff, he found his way into TV and radio in Wales, DJing and presenting, then hosting a quiz show. Moving to London, he became a regular provider of voice-over work on TV ads, but it seemed the nearest he'd get to being a household name was to provide the voices of Tango and Toilet Duck.
"I was getting to my mid-30s and it hadn't happened, and I'd been one of those people who everybody thought it would happen for. Everybody had always told me, 'Oh, you're very good.' I was making a very nice living making voice-overs. But I wanted more, and I was always told I was funny - always, always told I was funny. But could I sit at the top table with the big boys, as it were?" He affects a Moore-ish voice of creamy conceit: "Apparently I can." Whatever anxiety fuelled him through these years bubbled to the surface in Marion & Geoff. Brydon and director Hugo Blick co-wrote these sublimely painful vignettes about Keith, a devoted father whose wife had left him for another, pushier man. This was minimalist sitcom, verging on Pinteresque drama: 10-minute chunks of Brydon at the wheel of a car, talking direct to camera, creating an intimacy that survived even when the episodes were extended to 30 minutes in the second series.
Marion & Geoff was shot with Brydon alone in the car, except for Blick hidden in the back, with a camcorder on the dashboard. The series made him 70-per-cent better as an actor, he says, because the simple execution meant that for the first time he could relax and take risks.
"I know where the camera is all the time, I know where the windscreen is - very important, acoustically - I know where the mike is all the time. A bit like when I used to work in radio, in a way: you were in complete control."
Brydon comes across as a technician actor, intensely aware of the mechanical specifications of his work. He remembers shooting a scene of Human Remains on a beach with Julia Davis: "a windy day and a blue sky, and it terrified us, 'cause words were going - phhphw!" - he imitates gusts of seaside wind. "It was very hard to get any tension." He had similar concerns shooting the soon-to-be-released fantasy MirrorMask, where he speaks from beneath an origami visor and merges with an elaborate CGI illusion. "In the finished thing, we're in this vast cocoon-like cathedral. The blue-screen stuff is not that enjoyable, it's very hard to pitch a performance."
Sounding faintly abashed, Brydon admits he's not as hungry as he was: "It comes and goes in phases now." On one level, he's taking more risks: last year, he made a concerted stab at serious drama, playing an urbanely tense, cerebrally lascivious lead in the TV play Kenneth Tynan: In Praise of Hardcore.
But in other ways, you could say Brydon is getting safer, disappointingly so. Supernova was hardly cutting-edge, Brydon admits, although he ascribes its lukewarm reception to scheduling choices. He concedes, too, that he could be accused of taking liberties with Keith Barret, in making him a smart(ish), confident TV host, but he's also very attached to him: "I find the character so easy to inhabit. I'd grown up on Barry Humphries, and I saw the chance to have my own Edna. I find I can think as Keith, on my feet."
Quite how much Brydon resembles his creation is hard to gauge. While explaining how he'll use certain ideas in Keith's stand-up show to entertain an older, more conservative audience, Brydon bizarrely segues from Keith joking about hoodies and teenage binge drinking into his own curmudgeonly jeremiad on modern life: "Now I genuinely - Rob - I am concerned in a vaguely Daily Mail way by the whole thing of hoodies. That's something that as a parent concerns me, and as a citizen it concerns me, and as a member of a community it concerns me. I'm very aware of the increase of rowdiness and lack of inhibition and suddenly it's OK to piss and vomit in the street, and suddenly it's OK to wear virtually next to nothing and be virtually shagging in front of people. Wow, how did this happen?" He pauses a moment, contemplating the horror of it all. "I think it's the combination of the Spice Girls and Mrs Thatcher, but that's just me. I'm joking but I really do. So Keith gives voice to parts of me."
One of those parts may indeed be quite close to Keith. When filming the first series of Marion & Geoff, Brydon had recently split up with his wife, which may have added an implicit resonance to Keith's longing to spend time with his two young sons, his "little smashers". Now Brydon and his partner Claire, an associate producer on The South Bank Show, live near his children - two girls and a boy - and he has their moral welfare in mind. Despite his involvement in Little Britain, he doesn't like them to watch it: "They know it, because they know David, and they see the billboards: 'Why is David wearing a dress?' But no, I think it's too sexual myself." He disapproves intensely of children being exposed to EastEnders: "The psychological violence!" But Brydon admits, "I'm Strictly Come Dancing rather than X-Factor. I'm a bit of a goody goody. I'm Blue Peter rather than Magpie, and Swap Shop rather than Tiswas. When Steve [Coogan] was listening to the Stranglers, I was listening to Shakin' Stevens," he says, proving his point somewhat: it takes a true innocent to imagine the Stranglers were in any way more louche than Shaky.
It's largely because of his family, Brydon says, that he's not tempted to pursue an American career. "No thanks. Ronnie Barker never did, David Jason never has. Sellers did, but was he a happy man? I've never been over there and not felt guilty for being away from my children." At least while he was shooting Supernova in Australia, he says, the children came to visit him; they enjoyed it so much that they're badgering him to do another series, so that they can go again. Brydon grins his broadest grin: "'Yeah, do it again, do it again!' Hih heeh!" he giggles with delight - and the resemblance to Keith Barret, chuckling fondly over his little smashers, is nothing short of uncanny.
'A Cock and Bull Story' is released on 20 January; 'MirrorMask' on 3 March