And he knows it. A tall, slightly dishevelled figure in rumpled blue anorak and white trainers, who shambles unrecognised into a smart Hampstead cafe, he laughs about his high quotient of screen village idiots. "Timothy Spall once said to me, `I wonder what it's like to play a character who isn't a wanker?' I told him, `You're asking the wrong man, because that's all I play.' "
Not that he minds. "Those roles are great fun to play," he observes. "As an actor, I'm quite prepared to look silly. I don't mind looking like a complete berk. It's a vital part of an actor's repertoire to be prepared to fall flat on your face. You've got to take risks, or you don't surprise anyone."
Broadbent certainly takes risks. The latest manifestation of this is his role as Peter Duffley, an incompetent bank manager in The Peter Principle, a new BBC1 sitcom. Any fears that the show would slip into a cosy, canned- laughter atmosphere are allayed by Broadbent's marvellously unhinged performance.
In the first episode, he is freaked out by a gay couple asking him for a mortgage. "The problem is," he flounders wildly, "we've completely run out of money." Eager thereafter to assert his macho credentials, he struts exaggeratedly round the office asking staff, "Did you see Top Gear last night?"
In creating his characters, Broadbent is not afraid to improvise. David Schneider, from Friday Night Armistice, plays a dim clerk in The Peter Principle. He praises Broadbent's ability to think on his feet. "Jim is instinctively in the moment," he reckons. "You don't feel he comes to rehearsals with a bag of ideas that he then hammers roughly onto the script. He tries things, and because his instinct is so good, they normally work. He makes you laugh each time he does a line and that's rare. On one hand, you're impressed, on the other, you're jealous."
Such off-the-cuff talents can push an actor over the top. Broadbent, though, is careful to remain within the realms of the feasible. "I'm always wary of going over the top. In rehearsals, I'll start off doing far too much and trust the director to say `Bring that back'. You try everything in rehearsal - that's when you can make mistakes. With something like The Peter Principle, it's a question of getting as close as you can to the line that's exciting without going over it. If it's not plausible, the audience don't care."
A consummate comedian, Broadbent is able to make his screen monsters cuddly rather than scary. Schneider acknowledges that the character of Peter "could be horrid, but Jim is such a nice bloke, he makes Peter likeable. If Jim was playing Mussolini, you'd think, `What a lovely, misunderstood guy'."
Peter is Broadbent's first leading role in a sitcom (he has never been allowed to forget that he turned down the part of Del Boy in Only Fools and Horses because of other commitments). "I liked that classic sitcom thing of the pompous oaf," he affirms. "Peter has a very grandiose opinion of himself that gets cut down to size. There have been quite a few of these characters, but I thought, `These people run our lives'. They're white, middle-class, middle-aged. What the nation looks for is to have these people shown up."
The other factor in Peter's pulling-power is that we can all identify with him. "As a race, we boast about our wonderful sense of irony, which other nations lack," Broadbent continues. "Because we're ironic, we can see there's an element of Peter in all of us. I think, `There but for a degree of knowing how silly I'd look, go I.' These people are like us; they just don't have an edit button."
In his time, Broadbent has worked with some of the world's top directors - Neil Jordan (The Crying Game), Mike Newell (The Good Father, Enchanted April) and Terry Gilliam (Brazil, Time Bandits). He still treasures his period with Woody Allen on Bullets Over Broadway. "I learnt lots," he recalls. "No one else in the world writes and directs a film entirely on his own terms every year. What's interesting is how he has set that up in America where it's star and money-based. The billings for his films are alphabetical and everyone's on the same pay. It's a benevolent dictatorship."
But the person Broadbent has learnt most from is Mike Leigh, with whom he collaborated on Goosepimples and Life Is Sweet. "He guides and manipulates a work, but you're part of the creative process. You're not just doing your own research, you're involved in the nuts-and-bolts structure. It demystifies the business of making films and plays."
In the coming months, Broadbent will be seen in major roles in big-screen versions of The Borrowers and The Avengers. But don't expect mainstream performances. "There is nothing more dull than being straight-down-the- line and not bringing any other angle. Whatever that angle is, I bring it."
The Peter Principle starts on BBC1 on Monday 2 June
Born in 1950, he was brought up in Lincoln, where his parents, keen amateur actors, often used to take him to the Theatre Royal. "I was aware of acting as an option from an early age," he recounts
After school, he went to art school for a year. "Then I thought, `If I'm honest, I might as well come out as an actor'." He studied at LAMDA
His first job was at Regent's Park Open Air Theatre, playing "Sprite" and "Sailor" in The Tempest. He did rep in Stoke, Ipswich, York and Chichester, before teaming up with the theatrical experimenter Ken Campbell (The Illuminatus)
He went on to found the National Theatre of Brent with Patrick Barlow and performed such jokey classics as The Greatest Story Ever Told and The Complete Guide to Sex
Among his other successes in the theatre: Habeas Corpus, A Flea in Her Ear, Kafka's Dick, The Government Inspector, and Goosepimples
Films have included: Bullets Over Broadway, Life Is Sweet, The Crying Game, The Good Father, Enchanted April, Brazil and Time Bandits. On television, he has had major roles in: Heroes and Villains, Murder Most Horrid, Blackadder, The Comic Strip's Detectives on the Edge of a Nervous Breakdown, Gone to Seed, and Gone to the Dogs