Wil Wheaton on paying attention:
To be a good, believable actor, and to create characters the audience can invest some of their own emotion in, I have to be very connected to the other people in the scene. This is because, in real life, though we all have our own set of expectations and existing experiences, when we are deeply involved in something with another person (an argument, or a passionate romantic moment, for example) everything slows to bullet time, and the rest of the world ceases to exist. In an acting performance, the audience expects me to deliver that same level of real-life focus when its dramatic counterparts arrive. If I do something that doesn't match up with something the other actor has done, the audience's built-in radar, which they've developed through years of personal experience, tells them that something just isn't right there. It hits them like a bad smell, and they tune out, because the actors have been caught “acting.”
Also, when two actors trust each other completely, are totally committed to a scene, and are really focused on each other, wonderful moments reveal themselves that would otherwise be lost if we just relied upon what is given to us on the page.
One of my favorite examples of this is from Almost Famous. Kate Hudson, as Penny Lane, asks Patrick Fugit, as William Miller, if he'll go with her to Morocco.
When she asks him, they've been running around a park together, and it's clear to the audience that they're falling in love. It's really charming to watch, and unless you're deeply cynical, it's tough to not smile with them, recalling the first time you fell in love.
According to director Cameron Crowe, Patrick asked Kate to ask him again, because he'd been staring at her, and just got lost in that moment, so he missed his line. But he was still in the scene, so he asked her exactly the way he would have if it had been real. Kate stayed focused on him, stayed in the scene, and asked him again, so we have this incredibly wonderful moment of two people falling in love that probably has many of you running to Netflix to queue it up right now. If either one of them hadn't been completely focused on each other, that moment (which would have been impossible to script) never would have happened. If we'd caught them “acting,” it would have ruined that moment, and the whole movie would have suffered as a result.
Those moments are magic.
I'd also give Cameron Crowe a gold star for this one, seeing that this unscripted moment was better than what he'd originally had in mind.