There are real trained squirrels in Charlie and the Chocolate Factory.
Having worked with trained chimps in Planet of the Apes and horses in Sleepy Hollow, director Tim Burton must've thought training squirrels would be a cinch! Burton originally requested 100 real-life squirrels to appear in Willy Wonka's nut room, but eventually had to settle for only 40. Senior animal trainer Mike Alexander and a team of four experts spent a total of 19 weeks preparing the critters for their film debut. Once the animals became accustomed to sitting with the trainers (each squirrel was trained one-on-one), they were taught to pick up a nut and place it in a metal bowl. The bowl was eventually replaced by a conveyor belt for the film.
Just as the movie squirrels have to sort between bad and good nuts, so too did the trainers have to sort between bad and good squirrels! Some squirrels had no interest whatsoever in picking up nuts. Others could grab the nuts just fine, but then they wouldn't let go of them! These "bad squirrels" were sorted into a second group and trained to run across the floor toward Veruca Salt. Only the smartest of the squirrels were able to perform the nut sorting trick!
The final version of the scene features a mixture of real-life, animatronic, and CGI squirrels to satisfy Tim Burton's original vision for a hundred-squirrel Nut Room. The next time you watch the film, see if you can tell the difference yourself! Or you could just read this article about the squirrel scene to find out.