The pair both joined the show as judges this series, and Demi said pop superstar Britney showed no diva tendencies.
"What surprised me the most about Britney is that she's actually really quirky," she told People.
"(It's) really cool to hang out with (her)."
Demi - who has been romantically linked to Irish One Direction star Niall Horan - added: "You kind of expect a legendary pop star (to) sometimes (be), like, a diva or something, but she's really not. She's a sweet, down-to-earth, Southern girl."