I've had a pretty interesting experience over the last year or so.
I've been working with another guitarist and he had been working on songs for a while, but all he had for any of them is the main guitar line. We recorded a bunch of them and I said I'd come up with a full arrangement based on what he put down - adding drums, bass, additional guitar lines, some solos, etc. We never really talked about what he was looking for, so I wasn't bound by any preconceived ideas. I just responded to what I heard.
Sometimes I gave him exactly what he was looking for, sometimes I took it in a direction he never expected. Once I've added my pieces he suggest tweaks and I'll tweak his tweaks, etc., but so far it has worked out really well.
This is my first real experience writing and it's been a good one. We haven't had much time to get together for a while so now I'm trying to write a few on my own. I'd like to see what he does with some basic ideas I give him (turn the tables, so to speak).