Playing Against Type: Robin Williams breaking free from his "adorable funny guy" persona. The director comments to Robin during the DVD Commentary that it's the first one Williams has ever done. Williams explains that he normally can't watch movies he's in, because he sees himself, rather than the performance. But Sy is so completely different from Robin Williams in every way that he can watch the movie without seeing himself.
Apparently, according to at least one "behind-the-scenes" documentary, not being the "adorable funny guy" was something that Williams had an incredibly hard time doing. As several Hilarious Outtakes can attest, Williams would often slip into spontaneity and goofiness when trying to deliver dead-serious lines. "YOU'RE WILL YORKIN!!!!!"