DEMI LOVATO joins the cast of "Glee" this season as NAYA RIVERA'S new LESBIAN LOVER.  And Naya says she broke the ice with Demi before filming began . . . WITH HER MOUTH.

She told "Access Hollywood", quote, "I made out with her in my trailer before we shot the scene, so I just warmed her up."

Not surprisingly, Naya's boyfriend, rapper BIG SEAN, was upset, that he MISSED IT.  Naya said, quote, "He was like, 'Oh man, I'm out of town.  I'm jealous.