I sort of co-GMed a reasonably short run-through of RttToEE. I had run the campaign previously for the guy who was going to be GMing it, and I sort of wanted to see how it went on the OTHER side of the screen, so I decided to play and co-chair, so to speak.
I had a character, but initially I went ahead and created him to be interesting but non-critical. A Dwarven bard smith/Item-Creation specialist. I didn't want to take a major role in the party, of course. I helped set situations up, debugged the game, helped the other GM set up encounters. We'd occassionally step out of the room and discuss something upcoming, putting together some additional RP potential. He would occassionally change something to throw player-me a curve-ball in a combat situation, say. I would take over if he had to be absent, I'd also run PCs of missing players.
I think it worked out well. Eventually the mild dis-cohesion of the group started to wear on things, players had to drop for various reasons, and I found myself retiring the bard for a more critical alt-front-line role as a dual-weild ranger/fighter. Unfortunately I eventually became the MAIN tank for the party and things sort of stalled out when it became apparent the group wouldn't be able to pass many encounters for lack of a major damage-beasty.
I thought it worked out well. It was nice to sort of play a bit, even if I largely knew what was around every corner.

((Which, BTW, is tough to do if you're not very adept at keeping secrets and have a great poker face.))