What is a writer?

From A.O. Scott's excellent review of the apparently banal "Something Borrowed:"

[John Krasinski] plays Ethan, Rachel's confidant, whose vocation as a writer is signaled by his taste in shirts and his habit of sarcastically pointing out what is already obvious. So when Darcy hogs the spotlight at the surprise birthday party she has organized for Rachel, Ethan astutely remarks that Darcy always has to be the center of attention. He will explain a lot more as the movie progresses, none of it especially surprising or mysterious. But I suppose that's just what writers do.