Set up your camera to the main character, and angle down towards the crowd. Make sure there are no faces among the extras that are too distinctive or distracting, or extras that are trying to steal the scene by over-acting. The extras should keep quite still, as your hero pushes through the crowd. This technique also underlines that your hero is an active character, somebody who will do things to the world, than have the world just happen to him. While everybody else stands there observing, he moves in and out of the scene. To the audience, this signals that we've met character who's going to take action throughout the film.