Let’s say your contract with one production outfit has expired with no available casting opportunities (whether tv or movie.) Does one walk in on a studio or an ongoing production? Do producers regularly hold auditions, open to both new talent and veterans? Would you say Hollywood has perpetual excess of acting talent?
They have agents that find them auditions for them.
As for excess of talent, well, depends on how you define “talent”. Yes there is a huge excess of people looking for acting jobs, but most of them are completely unemployable, acting or otherwise.
And no one told them?
^
I was reminded of that Shiri Appleby movie “The Killing Floor” wherein she tapped several aspiring actors in New York city.
I was in 3 movies
Hunt for Red October: The signalman flashing light. Volunteered to get underway on the Frigate
Flight of the Intruder: Sailor walking with a marine (supposed to be in the Philippines but actually we were in Long Beach Ca.) Volunteered
Rocky: A fan in the front row. We had to wear a tux which was provided.
I’m retired now
How many of you had to cram into one tuxedo? 
Haven’t you ever seen “American Idol” or “America’s Got Talent”?
There are some seriously deluded people out there WRT their performing arts talents.
And many, many people lining up to take their money who keep encouraging them. Success is just a new set of headshots away ($$$), or a different acting class ($$$), or a style makeover ($$$).
Check out the documentary The Hollywood Complex. It covers an apartment complex where many aspiring child actors live. Practically every other scene is with someone who wants money for something. And it’s really sad seeing the parents keep chasing the dream. All the sycophants keep telling them their kid is the next big thing, so they keep taking them to auditions, seminars, classes, photographers, etc.
Movies rarely hold open auditions for speaking parts. The closest they do is to issue a call for extras. That’s usually when they’re on location, though – in Hollywood, they get extras from Central Casting. It’s a far more efficient system; the main reason they do it on location is to promote the film.
Most movies use casting agencies these days. Actors register there and the production will give them a list of what they’re looking for. The agencies have actors and their agents on call and figure out who seems like a possible fit, and sends those to an audtion.
Of course, once (and if) you become established, your agent can contact the casting agency about possible fits for you, usually by knowing what movies are in production. And the next level is when the producer or director contacts your agent directly.
Except for Bill Murray. You just leave a message on his answering machine. If he likes it, he’ll show up on the set. He might even tell you he’s coming. 
For every Brad Pitt, Angelina Jolie, or Johnny Depp making ungodly sums of money in Hollywood, there are thousands of impoverished (and un-/under-employed) actors and actresses who believe that they are just one job away from living the dream.
It’s one of the things that makes the industry so dichotomous. Yes, there is epic $$$ to be made for a precious few people, but most careerists in Hollywood never see that kind of money. Hell, a lot of them burn out and transition into non-entertainment careers after a few years because industry employment is just so volatile.
FWIW, most of the lucrative jobs in Hollywood are in behind-the-scenes work that most people never see or even care about.
Something I’ve always wondered about the entertainment and music industries is just how difficult it is to be a working actor/musician and bring in a steady, say… 60-70k a year, like having a well paying regular job might bring in.
Is that a reasonable expectation for someone, or is it still more feast-or-famine than that? I.e. you’re going to make $200k minimum for being a steadily working actor, or you’re going to really be a waiter at Applebee’s who occasionally works in a car dealership commercial or as an extra in a TV show?
This has been the history of California since the Gold Rush. Precious few got rich mining for gold, either. But people who sold them equipment, clothing and the like got wealthy.
Were you an extra or a principal?
People who are truly unemployable probably won’t have agents. No agent is going to want to waste time with such a person, and no agent is going to want to send someone unemployable to a casting call.
Unemployable people trying to get non-union jobs, extra jobs, or agents - that I can believe.
I don’t know Hollywood, but I do know New York. There are places that hold “open” calls for kids - and always tell parents their kid is great (they specialize in babies) and direct them to a photographer for head shots. We went undercover to such a place. But if you stick to legitimate agents/managers this didn’t happen.
My daughter got her first job using a photo taken by a friend. She got head shots after that. Never had a makeover. That would be pointless - plenty of hairdressers on the set. Never took a for pay acting class.
I’ve been at plenty of casting calls for kids. I can count the pushy parents on the fingers of one hand. Kids go into the agent or casting director by themselves - it becomes pretty obvious if the kid does not want to be there.
I’m sure there are idiot parents on the fringes, just like there are yokels buying gold mine stocks on the fringes of the market.
My daughter did have an agent in Hollywood when we moved to California, but it was not worth doing auditions at this distance - plus she wanted to direct. 
And she did - in a summer arts program.
As for acting, if you can make $60-70K a year consistently, you have far exceeded the vast VAST majority of actors trying to make it. You are considered a “working actor”. That means you have occasional recurring supporting roles on one or more series’, a few scenes in a movie here or there, maybe commercials. You have a face that some people may recall.
But it may still not be enough - LA can be expensive.
This article from the LA Times about working actor Michael O’Neill (you might remember him from lots of stuff) is a little old, but it illustrates the struggle:
http://articles.latimes.com/2004/nov/28/entertainment/ca-actor28
Sounds like the answer to my question is “No”, and that it literally is feast or famine- superstardom, or likely making more working in any number of non-descript jobs.
Well, it’s not quite so binary. If you are an actor who gets cast as a regular on a series and it runs for a few years, you can go from being a Starbucks Barista to making $500,000 a year just like that. Not superstardom, but serious money.
Maybe commercials? If you want to make it, and don’t have delusions of grandeur, definitely commercials.
Unless you have a name you get paid scale no matter where you go. If you work a legit job you do a few days of work on a series episode, which runs maybe twice which reduces your residual rate. You get the same pay for doing a commercial, but it will run dozens or hundreds of times.
If you have a recurring role on a series you are already in the elite, credited or non-credited, since you work pretty much every week.
In the outside world the prestige is doing legit. In the business the prestige is getting gigs.
You also have a good metric about how well you are going to do. In New York anyway one job per 10 - 20 auditions is pretty good. If you are not getting sent to auditions by your agent you’re in trouble. If you get fewer jobs than this (and even fewer callbacks) you are in trouble.
You are still too binary. Consider the number of commercials versus the number of series episodes. There are a lot more series being filmed today than 20 years ago, but still no where near the number of commercials.