We may act partly because we love the attention, but beginning is usually the high point of nerves and for some actors there is real fear in it. So it’s worth acknowledging the difficulty of starting. Despite how precious and luvvie it might sound, to begin is to expose yourself: all eyes will be on you as you, effectively, announce that you have something to say. Things may not come out quite right: your voice might squeak, you may find your hands performing unexpected and unconvincing gestures, your delivery of the lines may lack conviction and so on.
On stage usually only one person has to take the decision to begin and once the first line has been uttered, or the first action taken, the play is up and running and the path of least resistance is to keep going. What’s more, because the audience is at a distance and still settling into the experience itself, any wobble on the part of the actor gets overlooked. But on screen every take requires you to begin. And the tiniest lack of commitment is utterly exposed, if only to the director in the cutting room. Many times I’ve searched in vain for a take in which a particular actor has nailed the opening half-second and truly hit the ground running.
It’s partly to assist with the difficulty of beginning that the director, or sometimes the 1st AD, calls ‘action’. (Although I know there are a few directors who say the equivalent of ‘when you're ready’ instead of ‘action’ and I assume this works for them.) In my book From Stage to Screen I use the analogy with jumping off a high diving board. ‘Action’ is the equivalent of the helpful push in the small of the back and before calling it I was always looking to see whether the actor was there, really in the drama.
So one way to respond is simply to jump when pushed. But if you are one of those actors who needs to take the decision to go for yourself – and that’s fine – make sure that when you go you really go. There can be no half-measures when it comes to the screen: commitment is all.