React BEFORE you Speak
In life, this is rather obvious. Someone speaking elicits a response from you and you react physically, emotionally, cognitively, facially, and then when socially convenient (or not) you react verbally. But because most people use the lines of the script to act upon, they’re really only acting when they’re speaking. This causes massive dead zones in their performance whenever they have nothing to say. You can still chase your task you can still have a tactic and often your ‘reaction’ is a tactic. Tactics are reactive as much as active.
The flip side of this is the actor who feels like they have to be filling each tiny moment of the scene with some kind of performance. This person is tiring to watch, they’re constantly performing in moments when simply staring would do fine.
Either way, the fake reaction looks fake. If you must ‘stage’ the reaction because you can’t find a way to do it naturally, then you do need to think about the type of tactic involved in the reaction and then wait until your instinct tells you to react. But don’t listen to what they’re saying, wait and watch for a moment to react off what they’re doing. You’ve probably heard what they’re going to say a few times already, and you know which part the character might react at, but then ditch that for the best time to react. Obviously some times, the camera is not even on you, but that’s fine – you should treat it the same, so the other fella has something to work off still in their shot.
Don’t be afraid to react while others are speaking, if you sit waiting for your line, you’ll be horribly dead before that, probably even experiencing a premature ejaculation of your words. Be in the moment, watch the other character, find something physical to respond off and don’t expect it to be there the next time they do it, on stage or screen, it’s the same. You need to respond when the moment strikes, whether you have words to say or not, and even if you have to wait a full minute, brimming full of ‘attack’ or ‘coax’ or ‘reject’ before you get to say the line, then be that, chomping at the bit, will only make the line more suited to the moment.
It might be helpful to try to watch the way that you respond to someone in a conversation today to teach yourself more about how and when you react.