Billie Eilish Opens Up About Living With Tourette Syndrome, Loving Justin Bieber on 'Ellen': Watch

That was huge, but if Eilish is being honest, when she was playing a show in London, Sporty Spice of the Spice Girls came backstage and that was...another level. When she was "growing up," Eilish said she watched the Spice World movie at least 40 times, not realizing it wasn't a fictional tale of a girl group of singers. So, when she saw the actual group perform, she thought they were re-enacting the film. "It wasn't."

She also discussed her Tourette Syndrome diagnosis, which she said she's lived with her whole life and which everyone in her circle knows about. "It's not anything different, you know? I never said anything because I didn't want that to define who I was," she said, explaining the strategies she's developed for helping to deal with the tic disorder that can cause involuntary, repetitive movements and vocalizations. 

"I think I've also learned that a lot of my fans have it, which made me feel kind of more at home with saying it," she said. 

Back to the pranks. DeGeneres noted that Justin Bieber recently started following Eilish, which the singer is still freaking out about, cautioning Ellen that she better not spring some surprise on her by having Biebs come out. But then, just when Eilish had let her guard down, a staffer in a Bieber wig jumped out of a box and scared Billie, who did not seem to love the joke, or get that scared.

Eilish also performed a haunting version of "When the Party's Over" from her just-released debut album, When We All Fall Asleep, Where Do We Go?, while sitting in a chair in the middle of a pool of water on a bare stage.

Watch Eilish on Ellen below.