Miriam Margolyes has worked at the RSC and in the West End; she has been touring her one-woman show about Charles Dickens and his female characters since 1989. Films include The Age of Innocence and Harry Potter and the Chamber of Secrets.
She doesn't see any difference between amateurs and professionals – so she would give her tips, such as they are, to anybody. The aim of any actor is the same: to tell the truth in such a way that people will be entertained, uplifted and surprised.
Listen before anything else. Read the text over and over again, and make sure you know the lines. Go and see other performances, and be critical about them: work out whether you'd have smiled in that place, or turned your head at that moment.
Never show off. You can sometimes come to a particular point in a show and think, "I'm really good in this bit." Never, ever think that. Never read reviews. She hasn’t read hers since she was in rep.
Never know more than your character knows. She isn't talking about research; she means that when you are performing, you must stay inside the truth of your character. Don't signpost to an audience what they should be thinking.
And the most important thing is to breathe. If you stop breathing properly, you get a sore throat. And if you stop breathing, you die.