I was in a similar situation my senior year of college. I was stage managing a mainstage show, when an actor dropped out. The director had me take over his role. The character was onstage the entire time, but had only 10ish lines in one scene, so I was able to maintain all my stage managerial duties during the rehearsal performance. However, I was not able to call my own show (because I was onstage), which definitely sucked. I went along with everything at the time because I wasn't sure if I would get a chance to act again while in school, but with the hindsight I have now, I would have fought it more. I would have much preferred beging able to call my own show instead of being onstage the whole time.