I run into question on the net about "reality" in screenwriting: how much is too much, what can you get away with, yada yada.

I've got a pet theory that might make it a little easier for nascent script writers to get a handle on the relationship between Hollywood and the so-called "Real World". Or not.

There is a reality that we all, more or less, share.

There is also a "movie reality" that we also share, and have expedtations of it just as we expect in our real life worlds that if we drop an apple it will hit the ground and if we punch our mothier in law we'll be won't get any that night. And truth be known, "reality" is little more than having expectations met, in predictability. We expect the sky to be blue, the sun to come over yonder, the big guy to punch you out if you grab his crotch, candidates to go back on their promises. Reality is basically what we KNOW to expect.

But things work different in the movie reality. Our expectations are different. We know the way things are, but they are different from what we know of ReallyWood.
We KNOW that it's a good idea to quit your job at your prospective bride's fathers brokerage and pursue our dream to be a jazz musician, because it will work out, you'll be happy and you'll meet a much cooler chick. We know that because that's what always happens.

In real life we know that would end in tears, but in the movie world there is no doubt at all that the nerd will get the cheerleader, the handful of rebels will defeat the massive foreces of the evil king, that the scumbag will give the swag to the poor because his girlfrind has a wholesome smile. We KNOW that in any contest we are watching, from chess to fencing to tractor racing, the woman will always beat the man.

I'm not talking about things like grenades blowing tanks fifty feet in the air or sixguns that fire a hundred times or wagon wheels that turn backwards. Those are production values, really. I'm talking about the fabric of interpersonal reality that we know, understand, and accept...but is totally different from the reality we know when we walk out of the theater.

A lot of really well-written scripts mess with our heads on this stuff. And it blows our mind. Part of the coolness of scripts like "Mean Girls" or "Juno" is that they keep moving us towards situations where we "know" what is going to happen...but something different happens instead.

The first time I met with the producer and director of my star-crossed TV series I mentioned an idea I had for a spin-off from it, a sort of Fantasy Island Meets Spiritual Healing sort of thing. The director--a hard-bitten Aussie--gave me a hard look and said, "Do you really beleive that crap?"
I said, "No. But anybody watching a film or TV knows that things work that way."

You're writing a scenario that plays out in that HollyWorld reality and it's well to have that somewhere in your mind.