Wanna know a great story/lesson in acting? Straight from a producer's mouth (second-hand, obvs):

I believe it's after the missle-drill scene, when a real fire breaks out. Denzel wants to cancel the drill, but Hackman keeps it going. And the result is, a cook dies from a heart attack.

So the scene is Denzel pleading his case on why that was mistake, and Hackman listens, and listens, and listens... and when Denzel is done, Hackman lays into him, and re-establishes his authority.

And the key to the performance is that Hackman has his eyes/eyebrows lifted open the entire time... but then just before says his line, he *drops* his eyelids dramatically... and suddenly his expression goes from 'friendly' to 'menacing'. Its perfect.

Now, to how that came about:

Scenes are filmed for practical purposes: you light the master shot, then slowly move the gear, lights and crew in for the medium shots, and then finally closeups.

Real actors save "the good stuff" for their closeups. And Hackman is a real actor.

So on the day, Hackman never did his "expression change" for the master or medium shots. He only did that facial move for the closeups.

He was essentially 'directing' the scene with his choice of performance, even though no one realized it during the shoot.