Britney Spears and Heather Morris

The lone Glee clubber who hasn't belted one out will finally be heard in the show's tribute to Britney Spears.

It's only too appropriate that it will be Brittany doing Britney.

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"At first, I was so stoked to sing. I was the only one who hadn't," says Heather Morris, who plays the delightfully ditzy cheerleader. "Then I went in the recording booth and I got so nervous. It was like, 'Freak! What if I sound bad?'"

In the episode (premiering Tuesday at 8/7c on Fox), Brittany will envision herself as the sexpot pop singer during the slithering come-on "I'm A Slave for You" and the dance-floor duet "Me Against the Music," which she'll imagine alongside Santana (Naya Rivera). (There's also a performance of "Toxic" with the entire New Directions team.)

It's all rather serendipitous, says the 23-year-old dancer-turned-actress; Spears is someone Morris "genuinely wanted to be" growing up. Working opposite one of her idols was "really, really cool" if entirely too meta to process.

"I just sat there and stared at her," Morris says of filming with Spears. "Like, 'Wow, I'm just hanging out with Britney Spears as Brittany the character. We're doing a scene. I'm singing her songs. It was weird."

Despite the nerves, her vocals "turned out awesome," she says. "Afterward, I slept for like three days. I have no idea what happened to me in that booth."

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Morris joined Glee as a dancer at the request of friend and choreographer Zach Woodlee. He needed someone to teach Chris Colfer and the rest of the Glee clubbers how to do the dance to Beyonce's "Single Ladies." The writers made her one of Coach Sue's scheming but mostly silent Cheerios.

"When I got the job, I was just the third cheerleader," Morris says. It occurred to her that the trio of Cheerios was not unlike the popular-but-evil threesome in Mean Girls. "I thought it would be funny to play her like Karen (played by Amanda Seyfried)."

Why Karen? For Morris, there is a big difference between Karen's earnest cluelessness and other dumb girls. "If you watch a sitcom and see a dumb girl, the actress is usually overplaying her dumbness. With Karen, she legitimately doesn't understand. She's like a little kid who says, 'I don't get it.' It's funnier to me to play Brittany as not super stupid, but really innocent."

Turns out Morris' take was completely in tune with what the writers wanted, and she was promoted to a full-time regular by the end of the first season. Morris remembers how emphatically Glee co-creator Ryan Murphy lobbied to lock her down full time.

"I was with my agent one day at some event, and Ryan came up and told us, 'We're gonna get her to become a series regular.' He just kept saying, 'We have to have you.' A week later, the deal was done."

"Ryan is my angel. Him and Zach. They're unreal."

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Busting out Brittany's one-liners has been a career highlight, Morris says. Her favorite? Brittany's question to Santana: "Did you know dolphins are just gay sharks?" The actress has big aspirations for it, too. "I really hope that I'm a Trivial Pursuit question. 'What character said this infamous line?'" (Of course, Morris also credits the man who wrote it for her, Glee co-creator Ian Brennan, who she says is "the smartest human being ever created.")

And soon, Brittany will do a lot more than just spout insanely hilarious zingers.

"We're going to get to know her," Morris promises. In Episode 4, viewers will get to go inside Brittany's bedroom, where she'll see plenty of action. Brittany will also attempt a real relationship in her own dim way very soon. "I'm excited because recently I've been getting to do real things. It's interesting to see how Brittany has emotions," Morris says.

Will she continue to sing? "I don't know. Maybe. I did pretty good with Britney Spears -- but they also have Auto-Tune," she says, laughing. "No, I did good. But I don't know if they'll have my character do any more singing. We'll see."