Demi Lovato and Britney Spears were "so nervous" on the American The X Factor judging panel initially.

The two pop songstresses were anxious about honestly critiquing contestants' performances on Simon Cowell's TV talent show at first.

Demi is happy to report that the pair are no longer shy about expressing their true opinions.

"We got more comfortable in telling our opinions without being afraid of the booing," Demi told People magazine.

"When Britney and I first started on this show, we were looking at each other like, "What do we do? We're so nervous!' "

Demi doesn't feel competitive with Britney.

The star confesses that she does occasionally butt heads with outspoken fellow judge Simon though.

"I don't have any rivalry with [Britney], friendly or not," Demi explained.

"I have a rivalry with Simon for sure. And you know what? It's not so friendly."

The X Factor two-part season premiere airs in the US on Wednesday and Thursday.