Credit: Jason Merritt/Getty Images; Splash News Online

Lea Michele couldn't believe her eyes when a super-fit Britney Spears walked onto the Glee set last week.

"She was so professional," Michele, 24, tells Access Hollywood. "She looked incredible. She's so tiny!"

Michele says Spears, 28, was "great to work with. It was one of those things where you go to work and you're like, 'I can't believe we're doing this!'"

Earlier this month, shots surfaced of Michele wearing similar duds that Spears wore in her famous "...Baby One More Time" video.

Though she confirms they shot a scene together, she says, "What we do together, I can't say. I will tell you it's incredibly iconic!"

Asked about Spears' episode -- set to air Sept. 28 -- Glee creator Ryan Murphy was mum.

"I can't say anything!" he told at the Emmys Sunday in L.A. "She is very, very sweet, and she was very well prepared."

He told Us she even played some cuts from her upcoming album, "which is really amazing. It's sort of a return to the sort of Britney dance stuff. There are some great, great singles on that album. I was really happy for her. I feel like she is coming back in a big way."

Added actor Kevin McHale, "She was super nice and willing to try whatever Ryan wanted her to do. I can't say what she did, but I can't wait for people to see it. I think it's going to be great."