American Idol alum Todrick Hall made a surprise appearance on The X Factor USA this week… as a backup singer to the contestants.

The ninth season top 16 contestant was first a part of Garcia’s rendition of ‘Proud Mary’ on Wednesday night’s episode before going  on to perform with nearly every contestant.

Hall landed the gig thanks to choreographer Brian Friedman.

"He's a great guy and a friend of mine who follows my work," Hall told The Hollywood Reporter. "He wrote me and said, 'I know this may be weird, but would you ever consider being a backup singer?' I was, like, 'Absolutely!' Two days later, I'm on stage singing with everybody."

Hall says he was most struck by Britney Spears: "It's weird to see somebody who's such an icon just sitting there, chewing her gum, or blinking or doing normal human things.

"It's a little bit alarming. I don't know how these kids are doing it -- sitting there, talking to her, being mentored... But I think this is an amazing experience. It makes me wish I had waited a little bit longer to do another show."

While he has no particular favourite, he ‘s a fan of a number of contestants on the show.

"Carly [Rose Sonenclar] is one of my favorites, because she's a Broadway girl. I also like Vino -- he was great on 'Let's Stay Together' and I loved his energy. I love the group Lyric 145 and Fifth Harmony who sang 'Hero.'"