What makes a "good" actor?
The obvious answer is that good actors give convincing performances -- you believe they are whom they're playing. My favorite male actors are Max von Sydow and Robert Duvall. Among women, I guess I'd pick Meryl Streep.
But there are other aspects of being a "good" actor. Perhaps the most-important is placing the quality of the film or TV episode above one's desire for attention or special treatment. In this regard, William Shatner is a poor actor.
Bette Davis is another example of a poor actor. Though she remains one of the great screen stars, * she's notorious for her resentment for lesser actors (Joan Crawford being the obvious example).
But she also treated Errol Flynn quite badly. In Elizabeth and Essex, Flynn has a difficult role, having to convey three somewhat conflicting emotional states at the same time. Davis publicly criticized him, then retracted her criticism.
Had I been Ms Davis, I would have said to Flynn "Errol, this is a difficult role for any actor. Let's work together to get the best-possible performance."
* She had the good sense not to turn down TV roles. I saw her recently in a Wagon Train I remember from childhood, playing a woman who thinks she's pregnant, but is dying from a uterine tumor. Her unaffected and gracious performance pretty much tramples everyone else's.