Christopher Masterson, who plays smooth-talking delinquent Francis on Malcolm in the Middle, tells TV Guide Online that there are two major hazards working on the set of the hit Fox comedy: falling objects, and practical jokes concocted by his TV bro

Erik Per Sullivan (Dewey).

"He seems like this sweet little kid, but he's thinking these demonic thoughts," Masterson jokes of Sullivan, who gained fame as the terminally ill Fuzzy in The Cider House Rules. "He'll pull some kind of prank on you within 10 minutes of your arrival."

The little terror has been known to slip a cupcake onto Masterson's seat just as he is about to sit down, or toss random objects out onto the set after the director yells 'Action!' "He also plays little mind games with you," adds Masterson, who previously had a recurring role on Dr. Quinn, Medicine Woman. "He'll ask you questions and then confuse you so much that you can't do your lines."

Still, Sullivan does have his redeeming qualities, Masterson concedes. "He's actually very funny," he says. "He's our little jester."

Regular pranks aren't the only occupational dangers on the show's soundstage, however. "On any set, you have to watch for falling objects," says Masterson. "I've seen lights fall out of the sky, like in The Truman Show."

Masterson, who also appeared on the big screen in My Best Friend's Wedding and American History X, has just finished shooting the VH-1 movie Back in Black opposite another reported troublemaker, Eric Roberts. "I had heard he was a huge character, but he was very down-to-earth and very professional and very cool," he says of Roberts. "I went out to dinner with he and his wife, and he was just the sweetest, normal guy. Maybe he calmed down at some point."