06/10/2012 at 03:00 PM EDT
is not the only one who knows how to play hard-to-please on The X Factor
During auditions in Kansas City – where Cowell was out sick – Britney Spears
proved she was taking her new job very seriously.
"She's got a really good instinct, and when she loves something, she looks like she wants to dance on the desk," a source tells PEOPLE. "When she doesn't like someone, she is incredibly tough."
Her tough side came out over the weekend, when she told one disappointed singer, "You have to be great to be on this show, so [I vote] no."
During another audition, Spears and the other judges – Demi Lovato
, L.A. Reid and Cowell stand-in Louis Walsh – walked off the set
when a contestant refused to face the panel as he sang.
But, according to the source, Spears has a soft side too, which she displayed when she heard one little girl with a big voice.
The young contestant, who sang "And I Am Telling You I'm Not Going" from Dreamgirls
, told the panel that if she won, she'd use some of the $5-million prize to take care of her family, so they would never have to be homeless again.
"Whoa! I don't even believe you're real," Spears, smiling, told the aspiring singer after a chills-inducing performance. "Was that just a track? You are amazing! Holy cow! You are a little woman, and you're going to soar."
As for Spears, since joining X Factor
last month, "she's really massively grown in confidence," the source told PEOPLE. "She loves interesting and fun auditions and is prepared to stick her neck out and support someone no matter if some of the other judges disagree."