When I made my living writing newspaper editorials back in the 1970s and '80s, the easiest and usually the best editorials I wrote were those I was passionate about, those where I knew what I wanted to say before I said it. The truth is, I wasn't nearly opinionated enough for the job, so I often started writing about a topic before knowing what I thought about it. I would put down the arguments for each side in my own words, and in the process of doing so one side would usually make more sense to me than the other. Then I would rewrite the editorial, or at least give it a new opening paragraph, to conform with my newly discovered viewpoint.
I love it when my own writing surprises me, when something I know I've written seems to have been written by someone else. It's a bit like saying to yourself, "Did I really just say what I think I said," after a moment of unusual candor. But I'm not talking about the things I regret having written, although there are plenty of those unfortunately. I'm speaking of things that make me proud, or would if I felt more responsible for them. Instead I feel more like the channel through which those ideas, which I don't remember having beforehand, were expressed.
Writing, more than just expressing one's thoughts, actually seems to often create those thoughts in the first place. Sometimes I've found I don't really understand what a book is about until reading my own review of that book. Then it becomes clear, or at least my own thoughts about the book become clear.
I came across the above quotation from E.M. Forster out of context in the Saul Bellow novel More Die of Heartbreak, so I don't know exactly to what he was referring. Do you suppose Forster's novels sometimes surprised him in the same way some of my editorials and book reviews have surprised me? I'd like to think so.