Is the "stage" only refer to the stage in the theater? Or does it also include a film set as well?

Native speaker’s answer
Rebecca
Yes, theatre originally started as live perfomances, so it was natural to call it "being on stage". However, you can also use it to refer to any film contexts as well. A similar expression is "be on film". Ex: She is a natural on stage. Ex: Although he has no experience being on film, he blew the casting crew away.