Refers to a person who appears on stage, in a film, on TV, or in a similar performance. Can be used for both professional and amateur contexts.
The cast of this movie is very impressive.
She became famous as a stage performer.
出演者 refers to any performer appearing in a show, while キャスト specifically denotes the group of actors cast for a particular production or an individual's assigned role.
出演者 refers to the real actors/performers appearing in a show, whereas 登場人物 refers to the fictional characters they portray within the story.
Compound of 出演 (appearance, performance) + 者 (person). The reading is standard on'yomi.