Unlike Smarty, Wheezy and Psycho, Stupid is extremely overweight. Stupid is pigeon-toed and wears a blue and grey striped T-shirt, a red beanie with a yellow propeller, and dingy blue and white sneakers with untied laces. His weapon of choice is a baseball bat with a nail hammered through the top.
He is so stupid that he is shown hitting himself over the head with his own weapon while laughing. He has one line in the film, when he excitedly informs Judge Doom that Toontown is on the other side of the wall in the Acme Factory, which he and Wheezy are drilling open. He is the first weasel to die after losing control of his laughter, and his body is seen clutching a lily.
Stupid is first seen exiting the Toon Patrol car with the other weasels. Later, he is searching Eddie Valiant's office for Roger Rabbit and when he starts laughing, Smarty throws a toilet plunger on his snout to silence him. When Stupid begins laughing again in the bar, Judge Doom throws a record into his mouth. Stupid then tries to stop Eddie from escaping but fails. Stupid is not seen again until the film's climax where he is put in charge of working the Dip machine's controls, and then later dies laughing.
While Stupid is clearly an idiot, he is seen working the Dip machine's controls, so he does have some intelligence. Also, he is very competent with his master's plans, though may not be intelligent enough to fully comprehend and may just be mindlessly following orders.
At one point in the movie, Smarty warned that Stupid will die laughing, as a way to get him to stop. Aptly enough, this is exactly how Stupid, Wheezy, Greasy and Psycho die later on.