If you have studied those two great theorists of modern theatre, Konstantin Stanislavski and Lee Strasberg, you will know that both of them set great store in actors using their own emotional memories to help them express their emotions in parts they are playing. Many actors find this helpful, but some drama training treats it as if it were holy writ and my contention is that it is only useful up to a point.
In fact, appreciating the extent to which it is useful requires little insight. It's blindingly obvious that, if you think about something sad, you will feel sad and you will probably look sad too. The same with happiness, fear, anger and all the rest of the emotions. And that's as far as it goes.
You see, when you're playing a part in a play, you want to concentrate and focus on the part itself and you don't want to be distracted by the memory of something that's not in the play, particularly if it's something you find personally distressing. Also, the play is not about you, the actor, it's about an imaginary character (and others) that may resemble you to a greater or lesser extent. Everyone is different: thank about the mix of emotions you have when you are frustrated in reaching some goal: you are angry, depressed, resolute, we laugh it off, we blame someone else. Everyone is different, so why should the actor be the same as the character?
You can learn a lot by looking inside yourself, but a great deal more by looking at the world around you and also by using your imagination and drawing from the text you are working with. To be fair to Stanislavski and Strasberg they both favour an all round approach like this, involving the actor's imagination. It is some of their followers who have promoted a lopsided view.
Some drama teachers have treated acting training as if it were some intrusive form of psychoanalysis and subjected their unfortunate students to a process of 'breaking down' before building them up again in a form the teacher prefers. (That is if the student, having paid their fees has stayed around to be so mistreated.) Why such institutionalised sadism could be considered either ethical or useful is a mystery to me.
So: emotional memory is useful, but only up to a point.