Average Salary: $51,880
Expected Lifetime Earnings: $2,166,000
Nailing down an "average" salary for actors and actresses is kind of like trying to balance a tank on a sewing needle (well, except if you get this wrong you aren't going to be crushed by a tank). That's probably why even the governmental Bureau of Labor Statistics only gives an hourly rate instead of an annual salary, even though knowing annual salaries is basically why the department exists.
That's because they don't know how often you will (or won't) work, just how much you're likely to make while working (paying) gigs. And with a spread of income from the minimum wage all the way to $90 an hour (and above), the answer is basically "we haven't got a clue" (source).
Among other "facts" the governmental organization in charge of knowing these things knows is that there are close to 80,000 jobs available for actors in a year (source). That sounds great, until you realize there are probably a million people trying to get those jobs, and some of them will actually book more than one.
Some will call them successful, others will call them job-stealing jerks. Your preference will probably depend on whether or not you're one of the job-stealing jerks.
To be taken seriously as a professional actor and not just a hobbyist, you'll want to join a union. If you aren't a card-carrying member of SAG-AFTRA (film and TV), Actor's Equity (theater), or the American Guild of Variety Artists (everybody else), you aren't going to get many well-paid jobs (source). These unions not only regulate how much you get paid, but also things like meal breaks, hazards, and overtime. Non-union actors, well, let's just hope your day job provides health care.
Can you make money without joining the union? Yes, but it isn't doing you any favors. Oh sure, you might book a regional commercial with a $3,000 buyout or get $100 a day on an indie film, but union actors get paid every time their commercial airs, or their TV show is syndicated, or their film is re-released on DVD—this is called residuals.
And when you're an actor, the best word you could possibly hear is residuals. It's the closest thing to a steady paycheck this career will ever get you.