I have just finished reading Clare Dudman's interview with Tibor Fischer, one of the funniest I have read for some time. I have been keeping an eye on The Keeper of the Snails, and if you don't know this blog, I highly recommend it. It's full of life. . . in every sense.
One of Tibor's points – "I don’t think any two writers work in exactly the same way. It’s not like making a chair, whatever Socrates says. I start with a vague idea, a character, a scene and I see what happens. I write as much to entertain myself as others" – reminded me of a conversation I had with another of our authors not so long ago, in which he told me he was not so much a novelist as a "cabinet-maker" and needed to have everything planned before he could set off writing something.
So – improvisatore or cabinet-maker? I think that writers should simply do what suits them best. My own way of writing is similar to Tibor's. I start with a general idea and then like to improvise. I do sometime take down extensive notes, but I like to interact with them and remain above them – keep a free-range mind about. I would find it boring to follow a detailed treatment to the letter, and just fill it with words.