John Lithgow, Neil Patrick Harris John Lithgow, Neil Patrick Harris

One of How I Met Your Mother's biggest mysteries (no, not that one) will be revealed on Monday, but don't remind John Lithgow how big of a deal it is.

"I didn't watch the show ... so I didn't know that it was such a big deal to be asked to play Barney's father," he tells TVGuide.com. "I didn't know it was a six-year running gag: Who is his actual father? I said yes to it before I knew about all this. This is very typical of me — I don't really know what I'm doing. Everybody's so excited. There are great expectations now. I'm nervous!"

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Lithgow makes his first of two appearances Monday, as Barney (Neil Patrick Harris) at long last comes face-to-face with his absentee dad — the man he once thought was Bob Barker and the man his mom, Loretta (Frances Conroy), had previously told him was merely "Uncle Jerry." After receiving the letter Barney wrote him last month, Jerry surprises his son at his apartment to forge a relationship. "It's explained why [there was no contact] for 30 years," Lithgow says. "There's a lot of longing and need on both sides, as it happens when a child loses a parent at a young age. On both sides, it's a wanting to make things right again. It's all very deeply motivated."

For Barney, it's also a matter of reconciling the real Jerry with the man he's imagined his dad to be his whole life. So what's the actual Jerry like? Well, he's married with a family in the suburbs, where Barney ends up visiting. Lithgow stays mum on further details about Jerry, but teases that Mother pulls one of its trademark switches and "you get one impression of this character and you find out you're completely wrong."

"There's a big switcheroo, so that was doubly fun for me to play. ... But it's a real crisis for Barney. And Neil's such a great actor that he's really able to plumb the depths of that, even in a comedy," the former 3rd Rock From the Sun star says. "I mean, it's the stuff of a drama, yet it's treated so deftly and so gently, it makes for wonderful comedy. One of my favorite movies in recent years is Flirting with Disaster, that great farce film about Ben Stiller trying to find his biological parents. What makes that comedy so brilliant is that it's a real desperate search. It's really important for him. It's always great when comedy has that core to it."

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Father and son eventually work on finding the different colors in their relationship and their common interests. And, no, suits are not one of them. "I don't dress the way he does. Only Barney can pull suits off," Lithgow says. "I haven't said 'legendary' yet either, but there are some physical similarities. I think we make a good father-son duo."

Speaking of similarities, the coincidence that Jerry has another life in the 'burbs — not unlike Lithgow's Trinity Killer on Dexter — is not lost on the actor. "There is that weird parallel, isn't there? This sort of double life!" laughs Lithgow, who won an Emmy for his work on Dexter to go with the three he won for 3rd Rock. "But Jerry doesn't kill anyone."

Although he wasn't actively looking for comedic parts, Lithgow says one big reason he accepted the Mother guest spot is because it's a "wonderful counterpoint" to his murderous Dexter role. But he first needed a crash course on the sitcom and watched a selection of episodes producers sent to him.

"I hadn't known what the deal was with this show. I didn't know what the title meant. I didn't know why Neil always had a suit on and no one else did. To tell you the truth, I thought Neil was a ghost or something," Lithgow says. "I didn't know what the deal was at all. You're talking to someone who's clueless about most things in life and one of those things was How I Met Your Mother. But they sent me all these episodes and I loved it. Then I found out from my son and his wife that it was their favorite show, period. It was their appointment TV. And I do know Neil. I'm a huge fan of his. He's such a talented guy."

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The love for Lithgow was reciprocated: After his first day of shooting, creators and executive producers Carter Bays and Craig Thomas asked him to come back for a second episode, which they started shooting last week.

"They said, 'You've got to come back. We've got it all figured out how to bring you back.' Worse things have happened by a longshot. I've loved it here," Lithgow says. "There's no slapstick [in the first episode], but this second one gets a little out of hand. ... So now I'm hoping before the end of the week they'll tell me that again!"