Hank Azaria and Simon Pegg in Run, Fat Boy, Run Courtesy of Picturehouse

Getting behind the camera for the first time, wasn't that daunting?
Yeah, it definitely was. I wasn't really freaking out until I realized just how little our budget was going to get us. [We were] about to shoot over 50 locations in the most expensive city in the world to film in, with a very small budget and only 35 days to do it. And I didn't really have a choice in the matter. And we have a marathon in our movie! That to me is 10,000 runners, 30,000 spectators. But when I looked at the breakdown of our distribution of extras, day-to-day, I saw on the marathon days I had 200 extras. And I'm like: "Has anyone read the same script?!" But it just means that you have to be incredibly prepared and have a battle plan. Every shot had to be cheated, so that it looks like Simon's running with thousands of people.

Didn't you CGI some of the runners behind him?
We did one CGI shot that was from his point of view from the bridge where you see this huge throng of people. And that was a crowd replication shot which I had second unit do. And that was our one big shot that sold the idea that this is a lot of people.

Thandie Newton doesn't necessarily spring to mind as an actress famous for comedy. What convinced you she'd be able to carry the comedic aspects of the film?
Well, I didn't know if she could, but I believed that she just hasn't had the opportunity. I thought, given how Simon and I work [together], we would create the best possible circumstance for her to prevail. No one had given her a shot to really be herself, and if you know Thandie at all, this is who she is. She's a lovely, funny, warm friend and mother and wife, and she's never had the opportunity to be funny and silly and warm and grounded. And more than anything, I wanted to have a great time shooting — I didn't want to deal with ego. I just wanted to have fun and have a relaxed set. And so Simon and Thandie and I went to dinner and I just knew that the three of us were going to have a blast. She's so game; she was just up for anything.

Why a romantic comedy? A straight-up comedy might have been a more obvious choice for your directorial debut.
It's a tricky genre.

You've got to tread lightly because you can so easily split audiences.
Yeah. I think it's almost an act of rebellion because I love the genre. I love movies like When Harry Met Sally and Broadcast News. I think sophisticated, character-driven comedies are a dying breed. You get those movies like Knocked Up and Superbad, which are a very different kind of style, I think. And I was determined to try to, if possible, re-invigorate this genre.

What do you think about the state of comedy now? With films like Superbad and Knocked Up taking over the box office, it seems healthy but at the same time it's evolving.
It is. I think it comes down to taste really, and I guess I like the more sophisticated comedies — ones that are really about characters and relationships and have great dialogue. The trend right now is kind of veering away from that. It's more about the lowest common denominator: a lot of profanity, a lot of kind of scatological humor. I like a good fart joke. Who doesn't? And in fact, we've got a good one in our movie. But I just don't think the whole movie can be about that. I need something more, personally.

Will you be directing again soon?
I hope so. I would love to do another one — I can't wait. But next I'm playing Kate Beckinsale's husband in a political thriller by Rod Lurie, who did The Contender. So that's the next on my plate.