I'm not sure where you're based, but in NYC many people in theater (actors, directors, stage managers and more) have multiple jobs they can use to support themselves (and our ridiculously high rents) in between theatrical jobs. Here are a few of mine, and how I counteract the "they won't hire me because I might disappear" issue:
Babysitting: I don't work for one single family full- or part-time. I work via a number of babysitting apps, and most of my clients are either in need of one-time care, or infrequent care -- many times it's a stay-at-home mom who is looking to do errands or go to a doctor's appointment without the kids in tow. I have a handful of repeat families, but no one is SOL if I'm not available because I have rehearsal or a show. I'm also very upfront about that when I begin a relationship with a family.
Dog walking / pet sitting: Dog walking follows pretty much the same method as babysitting. I work via an app, so I can accept walks that work for my schedule, and I can not accept walks that don't. For pet sitting (especially if I am asked to apartment sit for the duration), it's very easy. I can most likely still do a theater gig while living in someone else's apartment and walking the dog or feeding cats. But if I can't, then the pet owner will certainly have another person available.
Event work: I was lucky enough to be hired by an event planner (weddings, bar/bat mitzvahs, etc). I can work in the office doing prep for various events when I have time, but she doesn't depend on my presence, so I'm able to work or not work as needed - I even have done some paperwork for her remotely on a day off while I'm working out of town. I also work as an on-site manager during events. She'll ask me if I'm available for a handful of dates and will take me when I'm available, and move on to one of her other assistants if I'm not.
Website building: This isn't by any means a steady gig, but I've been hired to build a few websites for people. I make sure that when I plan out the timeline with them, I take into account any other time commitments that would detract from my time working on the site.
Computer help: My event planning boss is not great with computers or technology in general. And neither are a lot of her friends. She has recommended me to many of her friends for help with technology, paid hourly. I've transferred files to external hard drives, obtained lost WIFI passwords, set up new iPhones, taught one woman how to sync her iPhone, and more. I am able to arrange these meetings to be scheduled around my other commitments.
Online: I recently joined UpWork, and have done a few jobs found through their listings where I've done transcriptions from audio files. There are a wide range of types of jobs on there, including Remote Assistants, and other things that don't interest me. But it's good to know that it's there.
That's how I keep myself afloat between productions. And how I've avoided ever working in retail or a restaurant.
