In the British GQ magazine, Jonathan Heaf in "David Nicholls talks ONE DAY," talks with the novelist and screenwriter of ONE DAY. It's a great profile of the writer, from his early days attempting to be a novel, to his recent success as a novelist and screenwriter. (Also check out video with David Nicholls -- "ONE DAY: A Novel Becomes a Movie. ") At one point, Nicholls talks about the need to keep the roles of novelist and screenwriter separate.
When I write, I'm in no way thinking about the film. If I did that with One Day then it would be ten years rather than 20; and not nearly so many locations. Trying to squeeze 20 years into 100 minutes, that gives you only five minutes of screen time for each year and, of course, you can't structure it like that.