Sitting side by side, Simon Pegg and Nick Frost lay claim to the kind of easy banter that results from long-term friendship.

Pegg, wearing jeans, T-shirt and a baker boy hat, puts his arm around his mate’s shoulders at one point, proudly proclaiming: “Nick and I have worked together for 10 years and we’ve been friends for much longer.”

For cinema viewers, their off-screen chemistry has conjured up comedy gold, firstly with the small screen success of Pegg’s late Nineties TV show Spaced and then zombie comedy Shaun Of The Dead in 2004 and spoof police picture Hot Fuzz in 2007.

Now they’re hoping to repeat the success with their latest movie Paul, which marks the first time they’ve written a feature film together.

“Writing a film and then making a film is tough,” says Frost, 38, also dressed down in jeans and a T-shirt. “Yeah, we had a couple of creative barneys,” agrees Pegg, 41.

“In fact we were tearing each other’s throats out a second ago,” he jokes, shaking a fist at Frost’s face, before his friend continues: “The good thing about being best friends is it enables us to get over it, and not to let it eat you up inside until you’re an old man and living in a bedsit drinking yourself to death.”

Pegg explains the collaboration has proven an interesting experience. “We’ve slightly changed the dynamic of our characters in this one. In the other movies, which I wrote with Edgar Wright, I played the main character and Nick is the sidekick.

“This film is very much a double header. If anything, Nick’s character is slightly more dominant and confident, whereas mine’s a bit of a wallflower at first.”

Directed by Superbad’s Greg Mottola, the film revolves around an alien smart-ass named Paul, superbly voiced by Seth Rogan, who’s been locked up in a military base advising world leaders about his kind for the last 60 years.

When Paul worries he’s outlived his usefulness and fears the dissection table may be drawing uncomfortably close, he escapes and hitches a lift on the first RV that passes by. Fortunately for him, the RV’s driven by Clive (Frost) and Graeme (Pegg), two sci-fi fans who’ve been saving for decades to make their pilgrimage to America’s UFO heartland.

Frost describes the film as “a love letter to Spielberg and a love letter to fan boys”.

“There are lots of little nods to other science-fiction films and that’s because it’s a tribute to a certain kind of film, that kind of mid-80s Steven Spielberg, Joe Dante, John Landis kind of caper film.”

Spielberg does in fact make a cameo appearance in Paul.

‘‘It wasn’t the worst day of our lives,” says Pegg, recalling the day the legendary director, who Frost and Pegg have been working with on the big-screen adaptation of Tintin, requested a role in their movie.

“We asked him to read,” Frost deadpans. “It was between him and Roland Emmerich and Steven got it. He did the best reading – ‘This is Steven Spielberg reading for the role of Steven Spielberg’.”

Arrested Development’s Jason Bateman and Blythe Danner also star in the film – as does Alien star Sigourney Weaver.

Frost was Pegg’s best man when he got married in 2005 (he and his wife, Maureen now have an 18-month-old daughter) and Pegg returned the compliment when Frost married executive producer Mariangela.

The idea for Paul originated during the rain-sodden Shaun Of The Dead shoot when Pegg joked he’d like to make a movie “somewhere it never rains, like the desert”.

In fact they ended up shooting Paul in New Mexico, which Pegg described as “one of the most changeable places in the world”.

He says: “It would be blazing sunshine one minute and 20 minutes later there’d be hailstones the size of golf balls.”

They’d had a taste of life in this inhospitable climate during a road trip across America’s West before writing the script. So much happened to us on that trip that’s in the film,” says Pegg. “We hit a bird, we got slightly terrorised by some weird guys.”

On their return, Pegg and Frost watched more than 50 movies about aliens and road trips.

“Then we sat opposite one another and banged it out, line by line,” recalls Frost. “For a time, Simon went off to do How To Lose Friends And Alienate People, so I went away for a couple of weeks and wrote a big 180-page script. When Simon returned we deconstructed it, kept what was good and what was bad was elbowed, but we discussed every line, sometimes for hours.”