I think what makes a "good cast" is so broad and can be achieved in so many different ways that I wouldn't really know how to answer this, but I think the most important part of a good cast is the interactions that the cast have with each other. Characters don't exist in a vacuum, and their interactions are how we learn more about them. How they talk and act around others, and how that teaches us about them, is the most important part of characterisation to me, particularly in dialogue.