One mark of a world-class actor is the ability to convince us that a character exists outside the confines of the screenplay. It is one thing to skillfully deliver a dramatic monologue, to tap depths of volcanic rage or crippling grief for a big Oscar moment played to the cheap seats. Far rarer and more valuable are performances that quietly suggest what isn't spelled out: key character details expressed through gait or inflection; off-screen experiences hinted at with a look, a gesture, or a wayward smile. This is really a difference between an actor and a performer. Tom Cruise is a great performer. Philip Seymour Hoffman is a great actor. So is, for example, Patricia Clarkson.
In The Messenger, Ben Foster plays Staff Sergeant Will Montgomery, who is shipped home after an injury, and assigned to serve out the last three months of his enlistment on "notification duty" with the apparently unbalanced Captain Tony Stone (Woody Harrelson). The two of them are tasked with telling the next of kin that a soldier won't be coming home, a job that consists of knocking on a door, reading from a script ("the Secretary of the Army extends his deepest condolences..."), and bracing oneself for a likely onslaught of abuse.