One of the things I love about that NFL Films version of the Comeback Game (playoff vs. Houston, 41-38 in OT, for you youngsters who weren't there) is the close-up's of the sideline. You get a great view of the player intensity. Nobody needs coaches to get stirred up. All the coaching talk is adjustments and strategy. Nobody tells Talley or Reid or Reich they have to dig deeper.
That's a big reason I don't think speechifying on the sideline amounts to much. The best players bring it to the game themselves.