When Disney signed on Robin Williams for Aladdin, they found that he was also going to be voice acting in another animated movie—Ferngully: The Last Rainforest. The chairman of Disney Studios tried to pressure him into not doing it, and Williams angrily responded “It’s my voice! You can’t stop me.” (source)
The girl who played Robin Williams’ teenage daughter in Mrs. Doubtfire was kicked out of school for missing too many classes during filming. Robin Williams then wrote a letter to the principal ‘respectfully requesting’ he reconsider. She didn’t get back in, but the school did frame the letter.
Robin Williams agreed to be the Genie for Aladdin under the condition that his voice would not be used to sell merchandise. Disney did not hold up their end of the deal, overdubbed his voice for advertisements, then sent him a Picasso painting as an apology.