The person is always alone, and, if they're wearing a hat, there's a deadly weapon taking a prominent place in the movie and the poster. Intended to imbue the person with mystery and power, it also implies that they may be our defender as we stand behind them and let them do their thing. More likely, though, we're in for a movie with an improbably skilled or lucky hero who isn't governed by the same laws of physics as the rest of us. The poster may end up eliciting more emotion from you than the movie itself. Except for The Dark Knight. That movie was the bomb.