B.o.B, Hayley Williams, Bruno Mars Hit It Off At VMA Rehearsal

The trio start up impromptu sing-alongs during run-through.

When B.o.B, Bruno Mars and Paramore came together Saturday (September 11) to rehearse their 2010 Video Music Awards performance, the three acts were as relaxed as longtime friends ... it's just that the rehearsal was the first time they all actually met.

B.o.B and Williams' long-delayed introduction (the two recorded their top-five hit "Airplanes" separately and shot the video at different times as well) was quick and pleasant, then the two chatted it up like they were classmates hanging out on the playground.

Bruno Mars also met Williams for the first time; all three acts are labelmates through Warner Music Group.

The Hawaiian crooner was cool as a California night behind his piano during the rehearsals, busting out impromptu covers of Drake's "Find Your Love" and Cee-Lo's "F--- You," which he had a hand in co-writing via his production outfit, the Smeezingtons.

"Hey, hey, hey," Bruno Mars sang while he gently struck the piano keys. "I better find your loving, I better find your love."

Not to be outdone, though, B.o.B also got in on the act. While Bruno Mars keyed a "Nothin' on You" groove, the Atlanta rapper began his best Notorious B.I.G. impersonation, busting into Biggie's breakout hit, "Juicy."

"It was all a dream," he rapped. "I used to read Word Up! magazine."

The 27th annual MTV Video Music Awards will be broadcast live from the Nokia Theatre in Los Angeles on Sunday. The party starts with MTV News' VMA Pre-Show at 8 p.m., followed by the main event at 9 p.m. ET/PT.