The Paris Review has a fantastic series of interviews with authors and poets. This one with Ray Bradbury is great:

I discovered me in the library. I went to find me in the library. Before I fell in love with libraries, I was just a six-year-old boy.

I get the sense from his interview that he was a constant, regular writer, but not a very systematic one.

The short story, if you really are intense and you have an exciting idea, writes itself in a few hours. I try to encourage my student friends and my writer friends to write a short story in one day so it has a skin around it, its own intensity, its own life, its own reason for being. There’s a reason why the idea occurred to you at that hour anyway, so go with that and investigate it, get it down. Two or three thousand words in a few hours is not that hard. Don’t let people interfere with you. Boot ’em out, turn off the phone, hide away, get it done. If you carry a short story over to the next day you may overnight intellectualize something about it and try to make it too fancy, try to please someone….

I write all the time. I get up every morning not knowing what I’m going to do. I usually have a perception around dawn when I wake up. I have what I call the theater of morning inside my head, all these voices talking to me. When they come up with a good metaphor, then I jump out of bed and trap them before they’re gone. That’s the whole secret: to do things that excite you. Also, I always have taken naps. That way, I have two mornings!

He also talks about rereading his old work:

Every so often, late at night, I come downstairs, open one of my books, read a paragraph and say, My God. I sit there and cry because I feel that I’m not responsible for any of this. It’s from God. And I’m so grateful, so, so grateful. The best description of my career as a writer is “at play in the fields of the Lord.” It’s been wonderful fun and I’ll be damned where any of it came from. I’ve been fortunate. Very fortunate.

I heard Bradbury speak once– I still have an autographed copy of The Vintage Bradbury on the shelf above the table in the living room where I write– and he talked the same way about his creative process. He seemed genuinely to believe that his books weren’t his, but rather his muse’s. We all talk about the muse, but for many of us it’s a metaphor for the unpredictability of the creative process; for Bradbury the “self” that wrote stories was a really different person.

Make sure you read all the way to the end.