"Effort is sometimes underrated as an actor's tool. There is an inclination, perhaps a defence, to consider an actor's talent herditary or God-given; a 'gift' that one has or one hasn't and there is nothing to be done about it. Nothing could be more deceptive. Heredity, training, psychological background, and cultural experience might all be vitally important in the growth of the actor, as they may be in the development of a track star, but none of them outweighs effort; all they can do is make the effort easier and more spontaneous. Effort, powerfully and effectively aligned with the character's situation, is the directive of acting energy. It is the dynamic of stage presence."
-- Robert Cohen, in Acting Power
This is best understood if "effort" is not confused with "tension" or "strain."