During a conference call with reporters Thursday, Cowell, 52, opened up about why he thinks the "X Factor's" new judge, 30, might be even meaner than he is.
"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 said of working with the Grammy Award winner. "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."
According to Cowell, Spears "was kind of fearless" during the nationwide audition process. "She's going to surprise a lot of people, I think, when you see her."
Cowell added that Spears "has taken this very seriously" and isn't afraid to speak her mind. "In the past, we've had judges who never say no. She can't say no quick enough on this show. She's quite difficult to please. But she's a fascinating person and I got to like her a lot."
Calling fellow judge Demi Lovato, 20, "so amazing," Cowell said he's "especially" proud of Spears, who suffered a public meltdown several years ago.
"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 said of Spears. "She's very witty and funny and quirky, and I just have such a good time working with her."
"The X Factor" premieres Sept. 12 at 8 p.m. EST on FOX.