John Lithgow as a Psychopathic Murderer

In a storied career that has included roles as aliens, transvestite football players and presidents, John Lithgow finds himself in a part he especially relishes: psychopathic murderer.

As Arthur Mitchell on the Showtime drama Dexter (Sunday, 9 ET/PT), Lithgow is the Trinity Killer, a violent serial murderer responsible for the deaths of 60 people.

"It's been a fantastic experience. I love the fact that he's so nondescript – that makes him all the more terrifying," says Lithgow, who spoke from London after completing a two week run of his acclaimed one-man stage play, Stories by Heart.

Lithgow, 64, wasn't a regular Dexter viewer before he was offered the key role for the drama's fourth season. But Mitchell's complex, layered character and storyline enthralled him. "I was on the edge of my seat," Lithgow says. "It unfolds like a long mystery novel, full of astonishing revelations that have tremendous tension to who (Mitchell) is and what he presents to the world."

Indeed, Lithgow's Mitchell isn't the cut-and-dry villain he seemed in early episodes, when he appeared to be the loner who since-departed FBI profiler Frank Lundy (Keith Carradine) was tracking. Halfway through the season, Mitchell was revealed as a family man, teacher, church deacon and community activist whose Hallmark-card existence masked his extracurricular madness and flashes of violence.

"I love the blandness of both his good and evil side. It makes him scary and creepy," says Lithgow, best known for his double Emmy-winning performance as alien-out-of-water Dick Solomon on 3rd Rock from the Sun.

Lithgow had portrayed killers and psychotics before the NBC sitcom ended its six-season run in 2001, including memorable performances as an escaped con who torments Denzel Washington in 1991's Ricochet and as Sylvester Stallone's foil in 1993's Cliffhanger.

But it's the attention from 3rd Rock– which endures in wide syndication – that has enticed a newer generation of fans.

"The one great thing about having a background in comedy is that it gives you an opportunity to surprise and shock people," Lithgow says. "It's fun to do something people don't expect."

Lithgow Almost Didn't Take Role

Lithgow also has helped pull some unexpectedly strong ratings for Dexter. Last Sunday's episode attracted 1.9 million viewers, Showtime's largest single-telecast audience since 2004.

Mitchell is shaping up as a something of a mentor to Dexter (Michael C. Hall), the police forensics expert by day/avenging serial killer by night. In their budding pupil-teacher relationship, Dexter thirsts to learn how a fellow serial killer can lead a double life as a husband and father, roles Dexter is struggling with as he moves from a stark loner's existence to heading his own family.

Hall relished Lithgow's presence on the set, which wrapped in mid-October after filming the season finale. It airs Dec. 13.

"His performance has such clarity and nuance," Hall says. "He brings such a wealth of experience, yet its coupled with such an effervescent sense of play and enthusiasm."

Despite several gruesome scenes, the two shared some laughs, Hall says. "We have a similar sense of humor. Sometimes, the more twisted or dark the scene, we'd burst into hysterical laughter. At times, we were both giddy in this relationship of two serial killers – just completely damaged goods."

Lithgow was at the top of a short wish list for the Mitchell role, but he almost didn't take it. He didn't want to break a promise to vacation with his wife, Mary.

"After we pitched the story, he went home and was on the fence about it," says executive producer Sara Colleton. "But his wife loves the show."

Colleton recalled the first day Lithgow was on set and, in character, went nude for a bathtub scene. "He walked in and dropped trou; not many people could have done that," she says. "He was a delight to work with. He inspired everyone to bring their A game."