The first thing of note is this: The first step in preparation is to learn your lines until saying them becomes a predictable reflex. And don’t mouth them silently; say them aloud until they become totally your property. Hear yourself say them, because the last thing you want is the sound of your own voice taking you by surprise or not striking you as completely convincing. If you can’t convince yourself, the chances are that you won’t convince the character opposite you, or the director. You’re your first best audience long before anybody else hears you. So don’t be an easy audience. Keep asking for more. I learn lines on my own; I never have anyone read. Nevertheless, I try to learn lines as dialogue, as logical replies to what someone else has said or as a logical response to a situation. You’ll never find me going down the page with an envelope, blocking out my own speech and revealing the one before because it then becomes nothing more than “cues” and “speeches.” If you haven’t grasped the logic of why you say a particular thing, you won’t say it properly or convincingly. And if your thoughts aren’t linked to what you’re saying, then you won’t be able to say a line as if you invented it on the spot. So you must be familiar with the whole conversation, not just your own bits. One of the most crucial jobs you’ll have as an actor will be to know what you’re thinking when you’re not talking.
I've found as I've got older that fluency is something I struggle with, that sense of being able to say the line without stumbling over a particular word. Caine writes here that the lines need to become a predictable reflex, your 'property.' He notes elsewhere in the book that the difference between working on stage and working in movies is that the stage actor learns more about his part as he goes through rehearsals, but a movie actor is expected to turn up on the first day of the shoot and get on with the job as though he'd been rehearsing for months. But I've found more and more that if I know my lines in advance, before I start rehearsal, I feel much more confident. Of course, if it's a big part and you've only been informed that you've got the part a little while before rehearsals start, then you may still be getting the lines under your belt once you start rehearsing. However, with this current part (which is a small one) I'm going to aim to have my lines learnt before we start rehearsing. It boosts the confidence enormously, and that's something Caine is all for doing.
He adds: Say your lines aloud while you’re learning them until you find what strikes you as the best possible expression of that particular thought. If there are plausible variations, develop them, practice them, too; but keep them up your sleeve. If the director rejects your brilliant interpretation, you’re not left in a blank state of horror. You've already imagined and prepared other reactions to demonstrate. And most important, you’ve allowed for some element of malleability in your performance. Give your best reading as if it were the only one possible; but your mind should be hanging loose enough to take a leap, if necessary. For the moment, go with the line readings that seem to you the most valid. It may take some doing, but once the thought process is right, the words will follow. So much of it really is a matter of repetition, of saying the lines over and over again until you’re sick of them; until someone can give you a cue, and you can say, feel, and react to the whole cycle of events, including those related to everyone else’s parts. That confidence is your safeguard against terror. Otherwise, in the tension of the close-up, when you’re standing there and someone is saying, “Quiet! Turn over! Speed! Action!” you may well go, “To bum or not to bim, that is the question!”
Of course the stage actor doesn't have anyone insisting you say the thing correctly from the word go, but some directors do make you tense, and if your preparation of the lines is sound, then at least you won't be forgetting the lines because some other aspect of the rehearsal has upset your equilibrium. Anyway, even as an amateur, it's good to have a professional attitude.
He also writes: It may sound like a contradiction, but you achieve spontaneity on the set through preparation of the dialogue at home. As you prepare, find ways of making your responses appear newly minted, not preprogrammed. In life, we often pick up the thought that provokes our next remark halfway through someone else’s speech. Thoughts don’t leap to the mouth automatically. We don’t interrupt at every occasion when a thought formulates itself; or, if we do, we don’t have many friends. Similarly, in a film script, your internal thought processes might well start articulating themselves long before you get the chance to speak. The script sometimes directs you to interrupt, but if it doesn’t, your thought may start well before you get a chance to respond. There may be a key word that triggers you during the sentence the other actor is saying. So pick up on that; form your thought and be ready to speak. For example: Other Actor: I’ve got to get a bus to Clapham—I’m already late for my date. You: You won’t get far. There’s a bus strike. The other actor doesn’t stop talking after he says “bus,” so you can’t get in and say your line at the actual moment of thought recognition. But when you hear the key word “bus,” from that point on you know what you’re going to say directly after he stops. You can show this by your reaction. And that bit of acting can only come from serious listening.
This is good advice altogether, but again the important thing for me is that achieving of spontaneity by being thoroughly prepared. And part of preparation is trying out all sorts of ways of saying the line, so that it doesn't sound stale.
There are a host of other things in the book that apply not just to film acting but to acting in general, and I think any actor could benefit from it.