There is no such thing as good acting. I agree that this is a bold statement. Bear with me.
There is no such thing as good acting. There are ROLES that show us talent, drive, watchability, etc. There are actors who often succeed at the roles they are given. There are actors who get cast a lot. (Which is a different animal entirely.) However, there is no UNANIMOUS DEFINITION for good acting. We all see talent in different ways and places. Let's break it down. (For the purpose of this exercise I'll be using examples from film for accessibility. However, this post was sparked by stage actors.)
1. Good acting can be achieved by playing a role that takes effort but appears effortless. I believe that it's easier to put your finger on it when you know or have seen a lot of roles played by the actor/actress -- we know what they are like in real life and we don't recognize them at all in the role. Case in point: Heath Ledger in The Dark Knight. The role of The Joker had to be physically exhausted with all of the ticks, the voice, etc. but he pulled it off without a hitch. He terrified and thrilled us all. And I would classify his performance in this role as good acting.
2. Similarly, when we recognize the actor, or have seen a lot of their work, we can recognize the types of roles they usually play and can identify when they are out of their comfort zone or IN a role that stretches them to their highest, most watchable potential. Good acting is something that we WANT to watch. Case in point: Vivien Leigh in A Streetcar Named Desire. She's unhinged within the confines of the script's world yet in control of the role at the same time. She's gorgeous to watch on screen and pulls you through every step of the story.
3a. We wouldn't be able to recognize Vivien Leigh and Heath Ledger as "good actors" without the roles they are tackling. Therefore, while good acting is the combination of many factors, good writing often produces good acting. Good acting is hard to come by when a poor script is at large. And when we see good acting in a show with poor writing, it is even more astounding to watch. You can cast Meryl Streep, Al Pacino, Kate Winslet, and Clint Eastwood in a film but if the script is uninteresting, their performances aren't going to be that noteworthy.
3b. How do we define "good writing?" For me, good writing consists of the following: the element of surprise (I don't want to know how the play ends on page one. I want to watch a chain of events), characters who definitively WANT something, REVEALS that we infer not ones that are spelled out for us, necessary dialogue (aka if the conversation isn't relevant to moving the plot again - get it out of there), and appropriate length for the subject matter/plot.
There are many other ways "good acting" can occur, but I think you're starting to get the point. What we refer to as good acting is NOT always consistent within an actor, it is the perfect storm of the right casting, the right writing, the right push, and the right watchability. I heard Michael Shannon speak in Feb. 2009 right after he was nominated for Best Supporting Actor in Revolutionary Road and he put it like this: winning an Oscar doesn't mean you're the best actor of all time. It's the recognition that in this particular role, you did justice to your craft and stood out among your peers. His remarks have stuck with me and therein lies my personal definition for good acting. It is a phenomenon that occurs with the right combination of elements. Not every good actor is set up for a good performance with every role he or she takes, but when it does occur, it is an incredible thing to watch.
I'd love to hear your thoughts. What is your definition of good acting?