@Esther11 - I agree with you that our language has too many words with similar meanings. I think the term "extras" works well for theater and movies. And the term "temp or temporary worker" should be used to refer to retail and general office workers.
The label "supernumerary" should be used only for workers who are tied to a company and have the knowledge and ability to step into a higher position when needed. This could be in corporations, religious organizations, or political situations. Former employees and retirees would be likely to want these kind of occasional jobs.