Rita Moreno Says Playing Norma Desmond in Sunset Boulevard 'Ruined My Knees Forever': 'It Was Brutal!' (Exclusive)

Rita Moreno has a piece of advice for Nicole Scherzinger, Sunset Boulevard's latest star: "Don't muck around!"

PEOPLE caught up with the Broadway legend to talk about her upcoming guest appearance on George Lopez and Mayan Lopez's hit NBC sitcom Lopez vs. Lopez, airing Friday, Oct. 25. And while discussing her newest role, the topic of her former role in Andrew Lloyd Webber's musical came up.

"I loved doing that show," says Moreno, 92, who led the London production from September 1996 to October 1996, filling in for a vacationing Petula Clark. "I loved every minute of it."

Sunset Boulevard, based on the 1950 Billy Wilder movie of the same name, follows fictional faded movie star Norma Desmond and the desperate attempts she goes to in order to get back into the spotlight. A diva in every sense of the word, Lloyd Webber's original production featured elaborate costumes, over-the-top glamor and a grand staircase in which its actresses descended multiple times a night.

"It's a killer role to play," Moreno says, noting that the part came with great perks. "When you play Norma, your nails get done every week, the makeup is fabulous, the gowns gorgeous. It's weird because it's very exaggerated, but it's a marvel!"

That doesn't mean it was easy. In fact, Moreno says she's had long-term effects from her short stint in the show.

"Oh, it's exhausting. And what makes it exhausting is climbing up and down those steps! I mean, I think I ruined my knees forever! I'm serious! I have terrible, terrible, painful knees," Moreno shares. "And in order to come dramatically down a big flight of steps, you have to go up some steps backstage! And that's what really killed my knees! It was brutal!"

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Also brutal were the costume changes in the original production, which Moreno says were "very frequent" and "so difficult," they required multiple dressers backstage to help.

"You didn't have one woman dressing you, you had four women," Moreno recalls of the process of switching costume designer Anthony Powell's detailed looks. "One is doing your arms, another is picking up your leg and putting a stocking in, and then picking up the other leg putting a stocking in. And remember — these are costumes, not just dresses. You're even changing your wigs! It's all very thorough and very detailed and you all have to just stand there because the moment you try to help them, it just delays them."

"And you had to change in a very short amount of time because once the orchestra starts playing, they never stop till the very, very end," she adds. "So when you're changing from one costume into another, all of this has to be done in the shortest amount of time you can imagine so you can get out on the stage again with a brand new outfit. It's nerve-wracking! Those changes are monumentally quick."

Luckily for Scherzinger, she doesn't have to deal with all that. In director Jamie Lloyd's acclaimed revival, now open at the St. James Theatre in New York City and styled as Sunset Blvd., the costumes have been stripped back and remain consistent throughout both acts.

Gone are the turbans, luscious robes and adorned gowns. Instead, costume designer Soutra Gilmour has dressed Scherzinger in a simple black slip dress — with no jewelry, shoes or any other accessories.

Still, the Grammy nominee and former Pussycat Dolls singer is giving a full-throttled performance. And Moreno knows the demands of playing Desmond are built into the bones of the show. So when asked any advice she'd give Scherzinger, Moreno's words hold true.

"Yeah, you better be strong. You better be healthy. Don't muck around, don't drink too much," Moreno says.

"It's a very demanding role, in every possible way," Moreno continues. "She's in every bloody scene, every scene! There's not one scene where the actress can rest for a minute. Norma goes mad at the end of the play! And you almost feel that way, because you do not stop."

Tickets for Sunset Boulevard are now on sale. Lopez v. Lopez airs Fridays (8:30 p.m. ET) on NBC.