(CBS) The hit CBS show "How I Met Your Mother" returns to the air with new episodes beginning Monday.

The whole gang will be back, and, coming soon, a special and unusual guest star -- Britney Spears.

Josh Radnor, who plays Ted on the show, told Early Show co-anchor Harry Smith the cast didn't know Spears would be part of the show until the last minute.

"Well, I just started hearing about it last week," Radner said. "Apparently, her people came to us and said, 'We saw there's a part of this kooky receptionist, and have you heard of Britney Spears?' And we had."

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Photos: Pop Diva Britney

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Radnor said, when the cast showed up Monday for the table read, "There she was!"

Spears will portray a wacky receptionist in love with Radnor's character in the episode, which airs March 24. Unfortunately, Radnor's character is in love with the receptionist's boss, a doctor played by "Scrubs" star Sarah Chalke.

During the show, the receptionist undergoes a transformation of sorts.

"Initially, she's wearing glasses and (is) a little mousey and, as she develops this crush, she gets a little foxier, tries to impress me," said Radnor.

Spears finished shooting her part in the episode Thursday night. Radnor said he was impressed with the pop star's professionalism.

"She was on time, she knew her lines. She's going to be really funny," he said. "I mean, I have nothing scandalous to offer."

Radnor also dished a little on what will happen to Ted during the rest of the season.

"Well, he gets in a little trouble this coming Monday on St. Patrick's Day, which is what I guess St. Patrick's Day is for," he said. "He's gone over to the dark side a little bit, to the 'Barney' side."

Barney is the womanizing character deftly played by Neil Patrick Harris.

"And then at the end of that episode, Marshall, his best friend, gives him a little talking-to, a little course correction," he added. "Ted gets back to not being like Barney."

TV personality Vanessa Minnillo guest stars in Monday night's episode.

As for Ted's relationship with on-again off-again girlfriend Robin, Radnor said anything can happen.

"I don't know. We kind of fell back into each other's arms on Thanksgiving," he said. "And I don't know what's going to happen with that. I mean, what I think was interesting, we were cast, and it's a whole red herring like 'Oh, turns out that it's not.' But the producers really liked us together so much that they were, like, 'Oh, I don't know what to do now. We already established this isn't the mother.' So I guess it's like life. Confusing. I don't know."

The show stopped production due to the writers' strike that lasted from November through February. Radnor said he kept himself busy during the unanticipated break.

"I moved out of my $800 a month sublet," he said. "It was actually traumatic. I was, like, 'I don't want to leave this place.' But I bought a beautiful place. So I moved. I had all this time to move in. And I traveled a bit. I did some writing. I kept pretty busy."

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