What is an actor that plays a principle role?

An actor who plays a principle role is called a lead actor or protagonist.

Here's a breakdown:

* Lead actor: The main character in a film, play, or television show. They typically have the most screen time and drive the plot forward.

* Protagonist: The central character in a story, often the "hero" or the character the audience is meant to sympathize with.

While "lead actor" focuses on the technical role in a production, "protagonist" emphasizes the character's importance in the story.

For example, in the movie "Avengers: Endgame," Robert Downey Jr. plays the lead role of Tony Stark/Iron Man, who is also the protagonist.