Sentence:  Most of the music circuses use <WSD>jobbers</WSD> in four to eight of the leading roles for each show. These <WSD>jobbers</WSD> are principal actors who are employed for one or two leading roles during the season. There is no difference between a star and a <WSD>jobber</WSD>, except that a <WSD>jobber</WSD> is not necessarily a "big-name" performer ... 
Meaning of the word marked with <WSD> tags:  An actor temporarily employed for a specific role, often in a touring company.