A novel gives a writer the illusion of control. By describing in the most minute detail, the novelist seeks to fix characters immutably.
Plays are different. Plays are a collaborative process. The writer creates, but someone else brings the work to the stage. As Damian Trasler put it (in a frustrated tweet):-
Possibly unpopular opinion:
No matter how detailed and carefully written, the script is only the beginning. It will be, and should be, further developed by the director and actors in production.
— Damian Trasler (@Dtraslerwriting) September 2, 2018
This does not mean that the director and actors should change the script, but they, not the author, have the responsibility for bringing it to life. At a simple level, the writer may have a very specific vision of the actors who should play the characters, but those actors may not be available. (There are exceptions. When contemplating the first production of The Real Inspector Hound, Tom Stoppard was asked how he saw the roles of the central characters. He described Birdboot as a Ronnie Barker and Moon as a Richard Briers. Director Robert Chetwyn managed to cast Ronnie Barker as Birdboot and Richard Briers as Moon. However, as far as I know, they were not available for all the subsequent productions.)
So, as a writer, how do you know that you are usurping the job of actor and director? The biggest clue is the over-use of stage directions. If a direction is necessary to make the plot work (telling someone that they need to be on stage, for example), then that’s fine. On the other hand, if you repeatedly describe facial expressions or tone of speech, then close down your script file and write your novel; that’s clearly what you want to do. I’m not saying never do it (sarcasm, for example, is not always obvious) in written dialogue, but if you keep describing tone and expression, you’re stifling the actor. Furthermore (whisper this quietly), sometimes the actor will find something that the author didn’t realise was there.
Similarly, if the writer stuffs a script with pauses, beats or ellipses, it becomes unreadable. The writer’s effort to show the rhythm of the speech gets in the way of the actors need to find that rhythm. (If it is essential to you to dictate the cadence of every speech, then turn your script into a poetic monologue and perform it yourself!) Of course there’s a time and a place for pauses. There are the pauses that indicate where the gaps are in a one-sided phone conversation, and there are the long, significant pauses that make the audience uncomfortable. In general, however, it’s far better for the writer to concentrate on punctuating the sentences and let the actor find a rhythm to suit the character.
Oh, by the way, that novelist’s illusion of control: it’s an illusion. No matter what you put on the page, a different image will be created in the head of every reader.