In The Suicidal Dog, a new BBC short film to be released in cinemas this week, the eponymous hound is so distraught about being castrated, he contemplates various methods of ending it all. In a pivotal scene, the camera focuses on a fairground cannon marked: "Captain Gunpowder, the human cannonball." It then pulls back slowly to reveal the dog sitting beside the cannon, sizing it up as a means of committing suicide. Moments later, we see him flying through air - and then landing safely on the bouncy castle on the other side of the fairground. The sequence has all the elements of the deft, daft brand of humour that has become its director's trademark. Paul Merton has made his directorial debut - and it's very far from a dog. It's a poignant comedy, full of visual gags that would not have disgraced a classic silent movie.

In The Suicidal Dog, a new BBC short film to be released in cinemas this week, the eponymous hound is so distraught about being castrated, he contemplates various methods of ending it all. In a pivotal scene, the camera focuses on a fairground cannon marked: "Captain Gunpowder, the human cannonball." It then pulls back slowly to reveal the dog sitting beside the cannon, sizing it up as a means of committing suicide. Moments later, we see him flying through air - and then landing safely on the bouncy castle on the other side of the fairground. The sequence has all the elements of the deft, daft brand of humour that has become its director's trademark. Paul Merton has made his directorial debut - and it's very far from a dog. It's a poignant comedy, full of visual gags that would not have disgraced a classic silent movie.

But what is most remarkable is that the man who has made his name as an improviser par excellence on Have I Got News For You and Whose Line Is It Anyway? has now proved he can cut it in a much more meticulous art-form - film-making. This is not a matter of just rattling out gags off the top of your head, but of sweating over a script for three years and then pouring for weeks in an edit suite over a single effect. Merton spent days, for instance, refining the reflection of a steam train in the dog's eye - a shot that only appears on screen for a split second - and worked for weeks on a key moment when the film shifts from black and white to colour as the dog enters the fairground. "I wasn't happy with that for ages", he sighs. "The quality of the black and white looked like a public information film Lambeth Council might have produced in the 1970s - 'Keep Your Bins Indoors At Night'."

In person, Merton is much cheerier than the curmudgeon who grumps his way through Have I Got News For You each week. That's an act, of course. "I took a lead from Buster Keaton, who was known as Old Stoneface", he reveals. "If you say something funny, but look as though you don't know it's funny, people laugh a lot more."

Perhaps buoyed by the whooping reception accorded to the premiere of The Suicidal Dog the previous week, Merton is on ebullient form when we meet. He bounces in wearing a fetching fedora - "I feel like I should be advertising port in this."

It soon emerges that the only surprise about The Suicidal Dog is that Merton didn't make it years ago. The man is simply steeped in cinema, and so he was particularly delighted to secure the services of Jack Cardiff, the 86-year-old cinematographer whose credits include The African Queen and Black Narcissus, for which he won an Oscar. "It had always been my ambition to make a film, but it was the old working-class thing of nervously holding yourself back. It was only me that was stopping me. Then I suddenly thought, 'hang on, I've done The South Bank Show. I'm allowed to make a film now, aren't I?'" The BBC agreed and stumped up the cash.

His conversation is peppered with allusions to film-makers. Here he is discussing that canine cannonball scene: "It's how Buster Keaton would have done it. You're letting the audience put two and two together. Jacques Tati said you should never nudge them in the ribs too much. If you're not talking down to them, it's a far richer experience.''

Merton has been a voracious film student from a very early age: "I saw The General when I was 13, and I came out of the cinema walking on air. The film had been made 50 years previously, and yet it was still funny. For me, films became the second-best thing to going to heaven - and they were a good deal more tangible. When a film clicks, it's a life-enhancing experience. That's what Buster Keaton did for me when I was 13.'' Some 30 years later, Merton feels just as passionately. He says he was so hyped up before the premiere of The Suicidal Dog that he couldn't eat for 24 hours. It's a touching admission from a man who for the past decade has specialised in ironic detachment.

Merton clearly loves being in control. "The great thing about it is that you are the one shaping what's being seen," he says. "It's so much more satisfying than acting. It's the difference between being just a member of the jazz band and being Glenn Miller. I love being in the position to say 'this is what I want' and then finding out four months later that I was justified. It's like setting improvised thought in stone."

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Now he has delivered his short-form calling-card, Merton hopes he will be able to work on a full-length feature film. But it would still have to be on his own terms. "There would be no point in me talking to someone in a suit in Hollywood. I don't want to have to convince people that 'moose' is funnier than 'antelope', and I don't want to have to deal with Hollywood actors moaning about the size of their trailers. While I was filming The Suicidal Dog, I had a nightmare that the dog would say 'get my agent, I'm not happy with the size of the trailer I'm pissing up against'. If you can't make a film without interference, then why bother?"

But isn't he worried that people might say "you're only a comedian, just stick to what you know"?

"If the film's a success, that won't be an issue", he contends. "In a way, it's amazing that more comedians don't direct films. Someone who has earned a living making people laugh should be able to make a film."

Merton talks about directing with the enthusiasm of a teenager smitten by his first love.

"Martin Scorsese says that once you're bitten by the film-making bug, it just gets into your system - and he's right. Keaton used to talk about his horror of having to work with Abbott and Costello as gag-writers because they didn't care about the rest of the film. If you aren't passionate about films, then you shouldn't be making them.

"Making this film has been the most enjoyable experience I've ever had. It's far more enriching than being a comedian", he says, before adding candidly: "I was never a great fan of my stand-up. That was drawing in charcoal; this is working in Technicolor."

With that, he's off. He dons his fedora again, and leaves with a flourish: "Better get back to selling port."

You can take the man out of comedy, but you can't take comedy out of the man.

'The Suicidal Dog' is released across the country with 'Chuck and Buck' this Friday, and broadcast on BBC 2 next year