I blame the cinema for one of the more annoying problems in live theater today. Films are created in a unique way. Someone writes a script and sells it to a producer. The producer turns the script over to a director, who hires the actors, and makes the film.
The director may shoot the script exactly as it's written. The emphasis on "may" is there for a reason. More often than not, the script will be rewritten, or simply ignored. It's not unusual for a director to have the actors read through a scene a few times, then tell them, "That's more or less what I want. Now, give me something like that, and don't worry about the words."
As a playwright, that sort of direction annoys me. As an actor, it can be fun, except for the obvious problem of trying to remember exactly what you said for matching shots.
Screenwriters have certain advantages over playwrights. They're paid a lot more, for one thing, and these days they continue to be paid residuals more or less as long as those movies or episodes continue to be re-run. But, in return for that, they have essentially zero control over what a producer or director does to their script. The Writers' Guild, the screenwriters' union, has a department whose only real job is to study re-written scripts and determine who gets the credit for what, and in what order, for the finished product.
As for the copyright, well, look at the closing credits. The screenwriter or writers are credited, but the studio is the copyright holder. This is why the Writers' Guild is a union. Their members are classed as employees.
Major stars also exert an influence on film and television scripts. If you're Tom Cruise, or Bruce Willis, you get to change lines if you don't like them. Some actors can even have a director replaced with another, who will perhaps give them more leeway in "fixing" the script.
I suppose this is all well and good for movies. The problem is, this attitude is starting to seep into live theater. There are directors who seem to think, "It's okay for Scorsese to change things if he doesn't like them, so obviously I should be able to do the same thing." If you call the director an "artistic director," it gets even worse.
Directors and playwrights often see the director's role in a play in very different ways. The director generally views himself (or herself) as the person who will somehow transform a lot of words into a vibrant, unique, and very personal piece of theater. The playwright, unless this is a first production, is likely to view the director as the putz who's trying to screw up his dramatic vision in order to be "creative."
First productions are an exception mostly because, for those, everyone expects things to be changed. Even the playwright can't tell if everything is going to work until the show starts rehearsing. When my play, Coming Out, was produced in 2013, we made a lot of changes. Some were minor, such as changing "euphemism" to "slang term" to accommodate an actress who found it difficult to say the original. Some were bigger, such as cutting several minutes from one scene. In one case, when the antagonist's wife is asked if her husband owns any guns, the actress unintentionally changed the line from, "Several" to, "We live in Texas, what do you think?" The ad lib is now firmly ensconced in the published script. The original was informative; the ad lib is funny.
But that's a first production. The director and playwright tend to work as a team on first productions, seeing what works and what doesn't, making changes as necessary. The play, and the credit, remain the playwright's. That's a basic rule. No matter who comes up with a change, if it stays in the script, the playwright owns it. This is the biggest difference between a playwright and a screenwriter. The playwright is the copyright holder, and from the moment the play is written, until 70 years after he dies, he's the only person who can change anything. The director can ask for changes, but only the playwright can say yes or no.
Stage directions usually offer some flexibility, particularly with older plays, where the stage direction in the script wasn't written by the playwright, but simply records what the original stage manager wrote in the prompt copy. Again, I say "usually" offers some flexibility. Shakespeare wrote almost no stage direction beyond entrances and exits. Many modern playwrights add only a little more, relying on the director to move the actors around the stage.
Other playwrights, Becket being one of the more obvious, include extensive stage directions, and expect them to be followed closely when the play is staged. Others, like Neil Simon, want every line to be delivered exactly as written. If Simon wrote, "that's right," the actor can't get away with saying, "that's true." At least, not without written permission.
More and more today, directors dislike this. They want the same broad allowances film directors enjoy to make a production uniquely their own. A stage director's job is to be interpretive, but too many of them would prefer to be creative.
If a stage director says, "I want to collaborate with the playwright in making our production fresh and relevant," the playwright is likely to feel the play is already fresh and relevant the way it is, and suggest that the director confine their collaboration to moving the actors around on stage and making sure they say the lines the way they were written. This is somewhat reinforced by experience, which suggests that the director's idea of "collaboration" is to do whatever the hell he wants to and not bother to tell the playwright.
Sure, sometimes playwrights can be quite flexible. I was in a production of Damn Yankees where the Senators became the Indians, at least partly because by then the Senators had become the Twins, the Indians weren't doing that great so the plot fit, and, more importantly, the team was willing to loan us the uniforms, saving a lot of money on the costume budget. That change was allowed, probably because it literally involved changing nothing more than the name of the team and a couple references to the location. Parma instead of Chevy Chase, or something like that.
Eric Idle sometimes seems to take a "do whatever you like, as long as it's funny" approach to amateur productions of Spamalot. Becket's estate, on the other hand, enforces his strictures on the interpretation of his plays. Edward Albee, and his estate, could be both liberal in allowing some changes, and astonishingly rigid when it came to others. With Albee, the question seemed to be, does it change the character in some fundamental way? If it didn't, he was known to provide the director with suggested adjustments. If it did, he'd put his foot down.
Too many directors today try to take a, "Just do it and hope no one notices" attitude toward script changes. Perhaps the silliest change I've ever noticed was a high school production of The Phantom of the Opera, where Monsieur Andre was transformed in Madame Andrea. That would have made sense if Andre was the character's given name, but that was Gilles. Andre was his surname, so whether the part was played by a man or a woman, there's no logical reason to change it. I find it hard to imagine anyone asked if it was okay to make that change, because the answer would almost certainly have been, "Of course not, it sounds stupid."
A few amateur and even professional productions have been shut down recently because a director went too far in trying to impose a personal vision. Scenes have been moved, dialogue rewritten, the sex of characters arbitrarily changed, and all without asking permission. If you're doing a production of To Kill a Mockingbird, there's always a temptation to tone down the racial language a bit. Yet much of the impact of the play depends on that language remaining intact, just as the play, and the novel, would have been far less effective if Harper Lee had kept the trial outcome from the first draft, where the whole incident was just a throwaway reference and an as yet unnamed Tom Robinson was acquitted.
Ultimately, if you're a director, it's very simple. If you want to change something, ask permission. If you want to make extensive changes, shift scenes, rewrite dialogue, or what have you, write your own play. Who knows? You might even turn out to be good at it.