The Dilemma movie billboard...

Mostly studios opt for the head shot route and rely on star power and recognisability of the actors to sell the movie, and on occasion they even get them to make a funny face (so that you know it's a comedy).
Here's such an example with The Dilemma movie billboard, featuring Vince Vaughn and Kevin James, snapped along L.A.'s Highland Avenue near Lexington Avenue on December 24, 2010.