John Lithgow was going to pass on the chance to play serial killer vs. serial killer on "Dexter," with an Italian vacation, cruise and his son's wedding awaiting him.

He made it to the ceremony but postponed the trip, and he credits his very understanding wife of 28 years, university professor Mary Yeager Lithgow, with pushing him into the plum job.

"She's the one who said, `No, you've got to take the role," he recalled.

Lithgow guest stars as Arthur Mitchell, the so-called Trinity Killer whose victims come in threes and who both repels and attracts Michael C. Hall's Dexter Morgan in the Showtime drama.

Mitchell fits the profile of those that forensics expert Dexter has judged deserve to die, but he's also a singular role model. Tempted to learn how the Trinity Killer has evaded capture, Dexter begins to make mistakes as he deviates from his protective "code," leading to a plot twist in Sunday's episode, "Slack Tide."

Lithgow, who has a nearly courtly manner and is deemed a sweetheart by colleagues, enjoys bringing the death-obsessed Mitchell to life.

"This man is very two-faced, like Dexter. A serial killer by definition is a man who presents himself in a very different way to the world," Lithgow said. "So it's an actor's field day figuring out what you reveal and what you don't."

It's difficult to push such a portrayal too far given the behavior of real-life murderers, he said.

"Every one of these stories that suddenly springs up in the news, you read it and think, `My God, how could this possibly happen? How could a human being behave this way?' _ which in a way is really liberating," Lithgow said. "You can't outdo the truth."

While "3rd Rock from the Sun" viewers fondly remember Lithgow as the egotistical, foolish alien leader passing as an Earthling, he's played more than his share of bad guys in movies.

The actor helpfully ticks off some of the titles, including "Blow Out" and "Obsession" for Brian De Palma, and "Cliffhanger," in which Lithgow adopted a British accent and particularly chilling demeanor.

Lithgow's last TV series was the short-lived comedy "20 Good Years," about two men trying to make the most of the time left them, a venture Lithgow called "very sweet and disappointing, of course, as any failed series is."

"I had a fabulous time with (co-star) Jeffrey Tambor, one of my favorite character actors. But it was a show for middle-aged old folks in a young world, so it made us feel kind of autumnal and melancholy," he said. Continued...