I get the sense Roger Ebert is very much into Synecdoche, New York. Following on from his review the other day, Ebert blogs at length about Kaufman in general and the film in particular, partly in response to Enertainment Weekly's review of the film - in which Owen Gleiberman gave the film a D+. And in response to a commenters' question, Ebert explains exactly how Charlie writes his screenplays:

Ebert: As a serious writer, he would no doubt refuse to answer that question. But you have asked me, not Kaufman, and so I will answer. He starts with the need to work. About "SNY," he has revealed, "originally, Spike Jonze and I were approached to do a horror movie." He stares into space. He solves a Rubik's cube with one hand behind his back. That inspires a plot. He visualizes some characters swooping in circles around that plot, wearing Jet Packs. They all look like Philip Seymour Hoffman or Catherine Keener. Then Kaufman starts writing hard as hell with no idea about where he is going, like an American man who won't use a map in Calcutta. He arrives somewhere. He parks his computer and strolls around a little to see where he is. He discovers some intriguing curiosities. He abandons the computer and takes a taxi back to where he started from. He drives his wife Denise and their kids crazy by complaining that he is a failure, he is almost 50, his hair isn't as curly as it used
to be, he doesn't know what the hell he is doing, and he thinks he should barbeque a chilled shrimp with his screenplay. Denise says, "Charlie, if you don't know what you're doing, I certainly don't know what you're doing. Why don't you go bother your twin brother, Donald? You're good at that." Charlie flies off to a film festival. This festival could be in hell and it would be an improvement. When he returns home he gets back into the computer, and tries to retrace his steps, knowing what he knows now. He fills up the tank with the curiosities, and starts writing like hell again, arranging and changing, placing carefully there a strange thing and a known thing here. He crashes into a wall. When he regains consciousness, he is amazed to find there have been no injuries and during the blackout he has discovered a miraculous somewhere he has never traveled, gladly beyond any experience, "where your eyes," he tells Denise, "have their silence." Then he sends the screenplay in to Harold Ramis' agent. (Source)

So now we know. Thanks to Laurel!