"We live on opposite coasts and never write in the same room. We spend a lot of time talking on the phone and emailing until we have an outline we're happy with. Then we divide up scenes, take cracks at them, and send them back and forth until we think they're good enough."
I enjoyed the film because it avoided the usual romantic comedy cliches, and the interview reveals that this was their purpose from the start, a film that is "more about the romance rather than some phony obstacle you know will be overcome by minute 85."
I should include the warning that doing this can create some resistance. I took a similar tack with a romantic comedy and the producer was totally behind it, but when he showed it to some of his colleagues they didn't get it. Their response was, basically, but a romantic comedy has to follow the standard formula..."
The quirky episodic nature of "(500) Days" also met some resistance at first: "The initial response was, well, it was interesting. Most people didn't know what to think. It certainly didn't look like the standard screenplay, with its big numbers in 20 point font taking up half a page. And it didn't read like a standard screenplay either, the act breaks coming at odd intervals, the story jumping all around in time, a dance number and a mini-documentary sequence appearing out of nowhere, etc. But a few people liked it enough to show it around and then a few more people liked it enough to become our managers and agents and from there it started to gain momentum."
(Collaboration is one of many topics discussed in my book, "Your Writing Coach," published by Nicholas Brealey. It gives you friendly guidance from idea through to publication. You can find it at Amazon and other online and offline retailers.)