It implies that the person will be taking action or engaging in a specific behavior, often in a theatrical or dramatic sense.
The phrase is commonly used in contexts like:
* Theater: "You are going to act in the play."
* General behavior: "You are going to act differently now."
* Responding to a situation: "You are going to act quickly in an emergency."
The verb "actuar" in Spanish is more commonly used in the context of acting in a play, but it can be used more generally to describe taking action or behaving in a particular way.