At 6 feet, 7 and a half inches, Moll's height alone makes him memorable, as Bull was meant to tower over the rest of the cast of Night Court, and Moll's height gave him an advantage as both Harry Anderson and John Larroquette are six feet, four inches. Though in his later projects he often has a full head of hair and/or a beard, his extraordinary height and distinctive speaking voice make him easy to distinguish. He is adept at playing both comic and menacing characters.

Moll had shaved his head for a role in a movie, but the producers of Night Court liked the look so much in his audition that they asked him to keep it. Moll also used the Bull persona in commercials for the Washington State Lottery Commission.

Moll made a guest appearance in the pilot episode of the Highlander television series. He played the evil Immortal named Slan Quince. He also made a guest appearance on Babylon 5 in the episode "Hunter, Prey" as a lurker criminal who was holding a VIP hostage.

In 1999's But I'm A Cheerleader, Moll went far against type and played a gay man who, with his partner (Wesley Mann), helps teenagers escape from a nearby "gay camp," where uncomprehending parents send their teenage offspring if they're suspected of being gay.

He can be heard in many animated film and cartoon productions, often as a villain with a deep, growling voice. Moll has voiced Bruce Wayne's disturbed friend and former Gotham district attorney Harvey Dent/Two-Face in Batman: The Animated Series and the Scorpion in Spider-Man: The Animated Series. His first role in an animated film was as a beat poet in Ralph Bakshi's American Pop.