Chemistry between the leads is obviously vital to a film like this. Were you sure you would have that with Zooey?
"The cool thing was that Zooey and I have known each other for a long time. We did a movie together called Manic almost ten years ago and it was a very different movie to (500) Days of Summer. It's one I'm really proud of actually. It's a very heavy, dramatic movie and we've stayed friends since then. You're absolutely right that the chemistry and the comfort and the trust between two people playing a love story like this is key. And to have a friend that I could trust and whose sensibilities I already understood made it so much easier I think. It was a big part of why it all looks natural on screen."
But weren't the love scenes a bit uncomfortable because you're friends?
"No, it's the opposite. It's so much easier when it's someone you know. It's weird when it's a stranger, but for us it was, hey, you know..."
The movie packs a lot in, but were there any 'Days' that didn't make the final cut?
"There used to be a sequence that was sort of the antithesis of the dance number. That one was The Best Morning Ever, but there used to be a Worst Morning Ever which was really funny and fun to do... There wasn't any dancing to it but they were going to play the same music, like happy music, but all these terrible things are happening."
The Best Morning Ever is a standout sequence, but it might've been easy to skim over it in the script. Did having to do all that dancing take you by surprise?
"Yeah, it took me by surprise when the day actually arrived and there I was in front of thirty choreographed dancers who were all doing the same thing as me. It was a bizarre experience because we all picture ourselves doing that, right? We've all sat and watched [Michael Jackson's] 'Thriller' and thought... Well, I certainly have and I never thought that would be me."
There are lots of interesting visual techniques, like the split-screen sequence showing the difference between reality and expectations...
"I think that the Reality vs. Expectations scene really gets at the heart of the movie, because here's a guy that's built his expectations on all the music and the movies that he likes and maybe what he's heard from friends rather than engaging with reality and being present. He's projecting all of these expectations to deify this girl and that's where he goes wrong."
Given all the post-production tricks, does the film look how you expected it to look?
"You know, I've got to say this one looks a lot like I expected it to look. It's what I hoped it would be like and how Marc (Webb, director) described it. Marc is very savvy technically. He's shot so many [music] videos he knows how to get the shot he wants, to make it look as it should. The surprise is that he's also an extremely humanistic storyteller, obsessed with story and character and not just making it look right, which is a double threat and it's rare. Usually you get a director who's one or the other, to be honest."
From the start we know the boy doesn't get the girl, but was there ever a debate about that with the studio execs?
"I don't think there ever was, no. I think that speaks to one of the many reasons the movie turned out well, because the priorities were in order. The director was in charge, not a bunch of executives on a committee and, you know, Fox Searchlight, who are putting out (500) Days of Summer, they also put out Slumdog Millionaire and The Wrestler and Juno and Borat, all these great movies. And they get it. They get that if you make good movies, respectful and dignified movies, then they can meet with a bit of success. They're outstanding and I've never really felt that way about a studio before, to be honest."
Are you a cynic or a romantic?
"Well, I think I'm a healthy balance of both. I probably lean more towards the romantic side. I never liked greeting cards much. I like to give something, but I like to write it myself."
You seem passionate about the indie film scene. Did you do GI Joe for the money?
"Actually, no! To be honest GI Joe is not the best-paying job I've had at all. I did that movie for fun because I got to do this cool character with a mask and this crazy make-up and the costume and the voice. It was a blast! I go in for diversity and an eclectic mix of creative challenges and GI Joe was really fun."
(500) Days Of Summer opens in UK cinemas tomorrow.