Remember all those walk offs that Britney Spears had in the early days of 'X Factor' filming? Turns out, she didn't have to tinkle after all. She actually had small meltdowns -- and almost quit!

"No one is going to admit that on the first day your highest paid star after 20 minutes has walked off set," Simon Cowell admitted in a phone call to TMZ. Cowell revealed that BritBrit "had a moment on the first day where she said to me 'I'm not sure I can do this.'" Cowell also talked more trash about 'The Voice' producers' premiere date tactics.

Thankfully, Spears kept the deal and her $15 million, because she was great at the job -- if much different from what Cowell was expecting. "You know how when you buy a dog, you expect it to lick you, and then it turns out to bite you? It was rather like that," Cowell told Us Weekly. "It was a real surprise. She's really, really difficult to please. But in a way I think it made it more interesting because she just wouldn't say yes for people to like her. And when you do these auditions in front of 5,000 people, they boo you."

Cowell said Spears "has taken this very seriously" and is "kind of fearless." "In the past, we've had judges who never say no. She can't say no quick enough on this show," Cowell laughed. "She's quite difficult to please. But she's a fascinating person and I got to like her a lot," he said. "I feel like the world hasn't really gotten the chance to see her personality over the last couple of years, and now this is the perfect opportunity for people to really see the light inside of her just shine," he added. "She's very witty and funny and quirky, and I just have such a good time working with her."