Lithgow has signed up to play Barney's father in the comedy but admitted that he did not initially realise that there had been some mystery attached to the role.
"I didn't watch the show," Lithgow told TV Guide. "So I didn't know that it was such a big deal to play Barney's father.
"I didn't know it was a six-year running gag - who is his actual father? I said yes to it before I knew all about 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!"
Lithgow also revealed that viewers will find out why Barney and his father have been estranged for so long.
"It's explained [why there was no contact] for 30 years," he said. "There's a lot of longing and need on both sides, as happens when a child loses a parent at a young age. On both sides, it's wanting to make things right again. It's all very deeply motivated."
He added: "You get one impression of [my 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 [Patrick Harris is] such a great actor that he's really able to plumb the depths of that, even in a comedy."
How I Met Your Mother airs on Mondays at 8/7c on CBS.
> 'Mother' renewed for two more seasons