Well...One of the things is that they get paid pretty well assuming they are getting equity minimum...which works out to just shy of $100,000 a year before taxes and not factoring in any additional negotiations or increments. (Link below)
Now, of course there are taxes, agents, managers, and just the cost of living in NY, but the wage allows for a certain ability to "float" between gigs.
Clearly not a TON, but between Broadway gigs are commercials, readings, concerts, workshops, Off-Broadway, Off-Off-Broadway, regional productions, tours, etc.
But is it possible for you to see a show and then see someone from the show waiting tables? Definitely...but you would be more likely to see a chorus or bit player than someone featured.
Equity Pay Rates
Baritone in search of a role in a new musical...