Sheila, revisiting a book by method-acting guru Lee Strasberg, talks about consistency as a primary virtue of the actor

Lee Strasberg was obsessed with consistency and could it be taught. Were there ways to HELP an actor to be able to repeat himself? Certainly there are. Techniques of relaxation and concentration are the bread-and-butter of any regular acting class, and – like an athlete working out every day – like a singer doing scales – like a ballerina at the barre … these things must become rote.
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actors need to know how to relax WHILE under stressful situations. Some people do it naturally. They come to life ONLY in clutch situations. These are the great talents. They always “show up” when it is demanded of them. They live for it. But many actors need to learn techniques. Relaxation is the key. Learn how to relax while surrounded by a jostling movie crew and you can relax anywhere.