'Superbad' and 'Knocked Up' star JONAH HILL is guest hosting "Saturday Night Live" this weekend, and ET's with him just days before his big debut!

"It's my life's dream, and it's come true, so that's pretty sweet," Jonah tells ET. "Of course I'm nervous ... but I'm just really excited to be here."

Songbird MARIAH CAREY is also stepping in as musical guest for JANET JACKSON, who had to bow out due to a case of the flu. Plus, we talk with CASEY WILSON, the show's newest cast member!