Final answer:
Makeup can transform appearance, indicate psychological traits, and make features expressive but cannot improve an actor's diction.
Step-by-step explanation:
Makeup in theatre has a variety of functions, but one of the things it cannot do is improve an actor's diction. Illustrative makeup transforms an actor to look like a character. Makeup can also evoke or highlight psychological traits and simplify or embolden features, making them expressive to the audience. In contrast, diction is related to the clarity and articulation of speech, which is not affected by makeup. While costumes, lighting, and scenery can also inform an audience about a character or setting, only vocal training and technique can enhance diction.