Whenever I am working with student actors at the elementary school where I teach, I have to spend some time going over what to do when a mistake is made during a live performance.
Young actors often think that what makes the difference between a professional and an amateur theatre production is that professionals are "better" and don't make mistakes. This, of course, is ridiculous. Whether we are beginners or seasoned professionals, we all make mistakes. The only difference between a performance that looks amateur and one that looks professional is the way performers handle the mistakes.
When a mistake is made, such as a technical problem or a forgotten line, the one thing that is most likely to kill the show (or at least the scene) is a break in character. A break in character may take many forms but is often given away by things like dead air/blank staring, apologizing or talking directly to the audience, talking to someone offstage, or (heaven forbid) some sort of hissy fit. A break in character is obvious to the audience because an actor has lost their focus. Generally, anything that draws attention to the problem will make it worse. For example, if a microphone is not working, stopping to stare at the microphone or (god forbid) try to fix it right then and there is akin to saying out loud, "Hey there audience, just in case you missed it, do you see that we have a problem with the microphone up here? Don't we look silly? " And although this should be obvious, never ever say anything about the fact that there is a problem. This includes things like apologizing or asking to start over.
By far the most common problem is a forgotten line. The solution for a forgotten line may come from the person who forgot it or a fellow actor. If you've forgotten a line yourself, sometimes very quickly saying the cue line again in your head will jog your memory. If you know where you are in the scene and are able to paraphrase the line, this will usually work, providing that your fellow actors will be able to get back on track in the scene based on what you say. If you remember another of your lines that comes very soon after, you may be able to skip ahead to that one (but be careful that you aren't throwing off your fellow actors). The most important thing to remember when a line is forgotten is to stay in character. Do something that makes sense for your character to do while you are thinking.
If someone you are performing with has forgotten a line, the nice thing to do is help them. An even better thing to do is to help them without making it look like you have messed up! One way of doing this is to feed them as much of their line as possible in the form of a question. Be careful though that you are including enough of their line in the question to make it obvious, because otherwise you are just hanging them out to dry by asking them a question they may not be able to answer. Or, if the line is not character-specific (it needs to be said, but could be said by anyone) you may just be able to say the line yourself.
And the number one thing to NEVER do?
Never, ever, apologize. It's live theatre, things happen.
