Back in the mid-90s Matt Costello and I were freelancing at Disney Interactive, scripting "MathQuest with Aladdin." The hope, of course, was that Robin Willliams would do the Genie's voice and he eventually agreed. We were back East when he came into the studio to do his lines, so we missed the recording session, but we were each sent a copy of the raw tape.
What a revelation. We'd assumed that the animators would choose the Genie's look in a particular scene and Williams would adopt the appropriate accent for the dialogue. Wrong! Williams did every single line in a different accent -- at least four per line -- and rarely the same mix of accents. At times he would stop and say he didn't like the way one came out and would repeat it until he got it the way he wanted. (I should mention that he was sick and running a fever during the session but this was the only window he had to be in the studio, so he hung in there like the proverbial trouper.)
The animators took the tape and chose the accent(s) they thought worked for a particular scene, and illustrated Genie accordingly. (Jonathan Winters also performed on "MathQuest.")
So there's my Robin Williams story: He performed dialogue I co-wrote. (How many degrees of separation is that?)