One of the bonus "easter egg" features on the Simpsons Season 3 DVDs is a hidden extra commentary track for the episode about a 300 pound white man in an insane asylum who thinks he is Michael Jackson. The voice for that character, of course, was provided by the real Michael Jackson.
The commentary, from writer/producer Mike Reiss, describes Michael as a very cooperative and helpful guest star, who even contributed several key story ideas. But I bet I'm not the only one who did a double-take when Reiss described this contribution: "Michael said 'make sure you have a scene where I stay up all night with Bart', so we did it."
Uhh, yeah. In case you think I'm making that up, here is the audio. Doesn't really mean anything, I'm sure it was totally innocent at the time, but it sure sounds creepy now.
