Kristen Stewart is Gonna Play Joan Jett

Kristen Stewart, who you probably know from Twilight or The Messengers, has been tapped to play Joan Jett in a new movie called The Runaways. Which begs the question, why do people say "tapped" as if you tap the actress on the shoulder and ask, "Hey wanna be in my movie?" I just don't understand some important things like that in this crazy movie business.