Speaking to People, Lovato explained that she didn't know what it would be like working with someone so famous.
However, she revealed that Spears is actually "really cool to hang out with".
"What surprised me the most about Britney is that she's actually really quirky," Lovato said. "You kind of expect a legendary pop star [to] sometimes [be], like, a diva or something, but she's really not."
Lovato, who described Spears as a "sweet, down-to-earth, Southern girl", also explained that the relationships between the judges are not supposed to be the focus of the show.
> Khloe Kardashian: 'My entire areolas were out on The X Factor USA'
"At the end of the day, it's about the contestants," she said. "It's not about us as judges. I think other shows maybe forget that the entire purpose of the competition is to find a superstar, not to get publicity for the wrong [reasons]."
Spears recently claimed that she will "definitely" be returning for The X Factor USA's third season next year.
Photo gallery - The X Factor USA movie week:
Copyright: Fox Ray Mickshaw