While the necessary skillset varies depending on the type of acting you pursue (think Broadway theater vs. TV commercials vs. film), all actors need a broad range of skills to portray the expressions and emotions of a character so effectively that they temporarily transport the audience to a world where the actor is indeed that character. This often involves:
Memorizing lines and stage cues
Analyzing scripts and scenes
Developing a character's emotions and physical expressions
Working with other actors, directors, and production crews
Adapting to unexpected changes and situations
Speaking clearly and adjusting one's voice to align with the character
Learning and developing different acting methods and techniques
Being comfortable in front of an audience or camera