Prosecution: Because you've seen Patch Adams, right? and Bicentennial Man and Jack and all the other thousands of crimes against film that this guy has been involved in. And aside from dumb comedies, which he does every time he needs a big paycheque, a lot of his dramatic work often hits that painfully earnest, oscar-baiting note. The kind of film that seems to care more about being rewarded then finding the work reward in itself. Dead Poets Society, What dreams may come and to a certain extent Good Will Hunting can all be counted under this category. He is entertaining comedian but can you really call this guy a great actor. Not unless he makes a lot more great movies very quickly.
Defence: Very few comedians can successfully make the transfer to 'dramatic' acting. Just ask Jim Carrey. But the Oscar winning Robin Williams seems to be one of the exceptions to the rule, and while the Oscars don't make great actors, Williams most definately is one of those even if you forget it from time to time. He can be hyperactive and over the top, but even at his absolute worst he is highly entertaining. And if you look at Insomnia and in particular One Hour Photo, he can underplay roles to supremely chilling effect. People still seem to see Williams as The Genie from Aladdin, all shouting and flash with no substance. But anyone who watches his films no that this is not the case. Look at what he brings to his role as a crazy homeless person in the Fisher King, aside from the crazy and the laughs there is a great deal of heart and he ends up stealing the film. Watch this film and you'll see that William is capable of more then you've seen him do in Jumanji.
Verdict: A tough one. Williams has made some bad films and given some bad performances, and they probably out-number the times he's been good. But when he's good he is a very distinctive actor and gifted comedian. So, although close I'm going to have to go with not guilty