'American Idol' Judges on the Secret to Carrie Underwood's Success (Exclusive)

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More than a decade after winning American Idol, Carrie Underwood returned to her roots.

On Sunday night's Idol, the country crooner served as a guest mentor on reality competition series -- which was recently revamped by ABC after being canceled by Fox after 15 seasons. Underwood kicked off her appearance with a rendition of her latest single, "Cry Pretty," but it wasn't just her fans who were elated with the performance. American Idol judges Katy Perry, Lionel Richie and Luke Bryan told ET they were "so happy" to see have the 35-year-old singer take the stage.

"She's so ethereal, beautiful, powerful, and it was wonderful to hear her say some of the things that we have yet to [say], 'cause we don't get to talk to these kids," Perry said of having Underwood help determine who of the two out of five contestants would be going home. "We just see them when they're on the stage and so she really helps steer their ships, and I think she's just a goddess."

Richie agreed, saying that Perry "took the words right out of my mouth."

"As far as I'm concerned, I just loved having a very famous alum," he told ET of the singer, who won Idol in 2005. "Alumni come back to the school and just tell us exactly how they did it and how they feel now. And she nailed it. Nailed it."

Welcome home, @carrieunderwood! 🙌 #AmericanIdol

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As for Bryan, 41, he thinks Underwood is absolutely "seizing the moment."

"Just her impact on music and culture and society -- she's [hosted] major country award shows for multiple years, been in [movies]... that's called seizing the moment," he stated. "I mean, at some point, that top 10, top five, top three... they will have to seize their moment. They can't just wait for the phone to ring. They gotta go [do it]."

Perry interjected, "There's a lot of people [who] have won that have never done anything with it."

Speaking of the final three -- Gabby Barrett, Caleb Lee Hutchinson and Maddie Poppe -- Perry and her fellow judges offered some sage advice to whomever comes out on top.

"In the beginning it's gonna be a whirlwind, and they're gonna be holding onto their wigs for dear life," said the "Chained to the Rhythm" singer. "Just be as pleasant as possible and take everything with a Himalayan grain of salt and trust the people that brought you here. And keep them around."

Added Bryan, "There is no pamphlet or tutorial, there's no Hollywood for Dummies. When something comes up, you just gotta be like, 'Oh my God, my life's over today, but alright we're back. And we got a number one single, and my life's over tomorrow, and we're back.' That's Hollywood."

"Ride it," added Perry, 33.

Richie, 68, chimed in that any future Idol hopefuls need not expect to take a vacation following their success.

"Let me just say, when the light turns green, there is no such thing as vacation," he explained. "Take it. Go, go, go. I can't tell you how many people have said, 'I got a number one record, I'm going on vacation, and you never heard from them again."

For more on the hit ABC series, watch below.

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