The essence of 'acting' is that you believe the actor is the character - their merging is seamless. That is 'acting'.

The talent and the skill are in making the audience (whether live or on screen) truly believe in and empathise with the character. If someone says 'it was well acted' they noticed it was acted. Thus, whilst it may be 'well acted' it is not great. If you watched Notes on a Scandal and felt Judi Dence entrenched in her character and you forgot she was Judi Dence, she has achieved a worthy performance.