AceShowbiz -

CBS has released the promo of "How I Met Your Mother" episode where Barney finally says, "Dad!". In "Legendaddy", when Barney finally meets his father, he's surprised to learn how his life turned out. Meanwhile, the gang points out each other's "gaps," something they should know by adulthood but never learned.

John Lithgow has made a transition from being a serial killer on "Dexter" to Barney's father on this sitcom. "It just seemed like a wonderful thing to do as a counterpoint to 'Dexter'," Lithgow told Zap2It. " 'Dexter' still resonates and hangs in the air. I thought it would be a great idea to do a lark-y thing like this."

Explaining his character's significance, Lithgow said, "He's the lost father. There's a pool of serious emotional stuff underpinning all of the comedy. [Barney] is what he is, in so many ways, because he was abandoned by his father. We make comedy out of that, but it's not necessarily the stuff of comedy. What's great is that there are moments in it that are extremely touching. They've set this show up in such a way that it allows for that."

Lithgow is set for two-episode appearances starting this March 21. "Ive just started shooting my second episode, and that's, as far as I know, all I'll be doing," he said earlier this week. "But I didn't even know I'd be doing the second one... I got the offer halfway through shooting the first. They may think of me again, and I enjoy doing the show so much, I would certainly do it."