The best advice my dad ever gave me is that acting is believing. Acting is not acting. It isn't putting on a face and dancing around in a mask. It's believing that you are that character and playing him as if it were a normal day in the life of that character.
You know when Hollywood does a great big blockbuster that really wraps you up in a world and lets you believe in extraordinary things that move you in some way in an almost operatic sensibility? That to me is the most fun I have at the movies.