"He has this thing where he tries to say everything that's coming out of my mouth," Lovato tells Radio Disney's Ernie D, laughing. "But yeah, he doesn't know what I'm about to say."
Lovato and Jonas, who performed the duet "Make a Wave" on American Idol Wednesday, met on the set of Camp Rock in 2007, and Lovato toured with the Jonas Brothers the following year. The two quickly became best friends.
Lovato, 17, admits that she and Jonas, 20, are accident-prone and have to avoid props like stairs when they're onstage.
"We're like the two clumsiest people ever," she says.
But when it comes to vanity, Jonas has one over on his girlfriend, he confesses. When asked who spends more time in the makeup chair, the heartthrob replies, "I do."