I had a couple scenes in License to Wed playing a singer in the church choir where Robin Williams's character was a priest. We filmed for three days at a church in long beach, and he managed to do a couple very cool things in that time.
1. If you've seen the movie, you know it's terrible. Not funny at all really. So we were doing these scenes in the church with Robin at the pulpit delivering these crappy jokes. As soon as the cameras would cut, he would start riffing and just busting up everyone in the room. It was hot and awful in the church and there were maybe 500 extras sweating away in their Sunday best in uncomfortable wooden pews for hours, and it could easily have been miserable for everyone. But he singlehandedly kept everyone entertained and happy.
2. Later we were filming with just him and the choir. Once again it was dreadfully hot and we were all wearing thick choir robes over our regular clothes. There was no A/C in the church, only big yellow hoses piping in cold air from outside, and of course they were all aimed at him. But between every take - and I mean every take - he would lift up the hose and slowly blow it back and forth over the whole choir while talking to us all and asking us questions about ourselves.
3. We had recorded the audio at a studio a few days earlier, so when we filmed we were just lip syncing to our own playback track. But the playback kept fucking up until they finally cut it and we were lip syncing to nothing. Not great. Still, it probably would've worked as we were all in the background and they could've edited it to look okay. But Robin said, "Didn't you guys sing this?" to which we replied yes. "So why don't you just sing it live now?"
The SAG rep had to explain that the rates for on-camera singing are a fair bit higher than off camera plus lip sync (to the tune of about $800 more per singer, with over 30 singers.)
"Fuck it," he said, "I'll pay the bump. Let's hear it."
So we sang it live and he loved it and it raised the spirits of the whole day and that's what made it in the film.
Basically he was amazing throughout the whole thing, and just a total fucking class act.