Steve Pratt talks to the talented team behind In The Loop, which looks like being the UK’s best comedy film in ages.

A government minister on an official trip to Washington decides to watch a shark documentary not a porn film on the TV in his hotel room in case it shows up on his expenses.

The line gets one of the biggest laughs, and sometimes a spontaneous round of applause, from audiences watching political satire In The Loop.

Did director Armando Iannuci and his writing team have some insider knowledge in the light of recent events involving MP Jacqui Smith and her husband’s viewing habits?

They didn’t. The line was written long before dirty movies appeared on her expenses claim. “You write all these things in advance and sometimes you think ‘will people think this is too silly and won’t believe this’ and sometimes you think it isn’t silly enough. You have to make a judgement on what you think is believable – and then these events happen,” he says.

“Steve Coogan has this line about a guy who wants to ban people talking in foreign languages in shops. That was filmed last summer and then there was a court case last month where that happened,” he continues.

“You start worrying whether the things we really made up about the war are actually going to happen, and whether that’s a good or bad thing. Then someone from government comes up to you ,after a screening, and says that it’s far, far worse in reality.”

True or false, In The Loop is the funniest British movie for ages as politicians argue over going to war in the Middle East. The film is a sort of spin-off from the TV series The Thick Of It, with which it shares a character – swearing spin doctor Malcolm Tucker, played by Peter Capaldi.

“It’s great fun obviously because he’s cleverer than I am because he has six writers who’ve written wonderful lines for him,” says the actor of the team behind the character. “He has a bigger vocabulary and is smarter than I am but I inhabit him, get the bile boiling and let him loose.”

Even an on-screen face-off with Tony Soprano, alias James Gandolfini who played the mob boss in the TV series, didn’t faze him.

The Sopranos star is cast as a US military man against going to war. “I’m a huge fan of James, so it was an enormous thrill when he walked into the rehearsal room,” recalls Capaldi.

“We had to have a go at facing off each other which was tough because I was inches away from Tony Soprano and he’s a big guy. But he’s brilliant. We just went for it.”

He even got to film in Downing Street. The makers of In The Loop wrote asking to shoot scenes there and, much to their surprise, were given permission. “The Prime Minister was away at the Labour conference in Blackpool, so Downing Street was sort of empty. You’d think there would be someone running the country but it was fundamentally quite quiet,” says Iannuci.

In the US, they were able to film Malcolm outside the White House (“they said where you could and couldn’t stand”) while actors Tom Hollander and Chris Addison had first hand experience of the Washington system while filming a scene in a limousine.

“We had this enormous limo that you’d normally expect to see a hen night or senior politician in,” recalls Addison, who plays a political advisor in the film.

“Tom and I were in the back, with Armando, the director of photography and various other people on the side of the car. And the car couldn’t move, we were trying to go through Washington rush hour.

“Eventually one of the outriders – they have to be real outriders otherwise it’s impersonating a police officer – knocked on the window. We turned it down and he said ‘do you want us to turn the lights on?’. For a good four or five minutes we slide through the Washington traffic. We were very giddy about the whole thing.”

But what a lot of people want to know is whether Malcolm Tucker is supposed to be Tony Blair’s spin doctor Alistair Campbell. Capaldi says that no one sat down and said it was supposed to be him.

“So the idea that it is him and that’s how he behaves is some strange melding of some concept of him and what we’ve done,” he says.

“I think people want Alistair to be Malcolm, but there was no great conscious effort to do that.”

■ In The Loop (15) opens in cinemas tomorrow