This is a broad and subjective question. However, I think it can be answered with advice such as: Don't include car chases, train accidents or other things that require special effects that can't be created using lights and sound.
closed as too broad by Standback, BugFolk, FraEnrico, Secespitus, JP Chapleau Jan 23 '18 at 14:21
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My advice is to try and act out the parts, actually say the lines, make the movements, and so on. If necessary, use cardboard boxes or other cheap stuff (empty aluminum cans as wine glasses or a lantern) for your props. Use a sheet of paper with a name on it, put it on the floor for other actors. Use your living room as your stage, or your bedroom or kitchen or garage.
In other words, you can do this very cheaply with imagination and dispensing with self-consciousness, to ensure your play doesn't present some insoluble or insanely expensive puzzle for a set designer, and isn't going to feel awkward to act out for your actors, or be too clunky and inauthentic to say.
That you don't give them lines or emotions that are too difficult, or stage directions that won't work, and that you don't leave them standing there like a mannequin while some other actor works through a three minute speech.
Actors want something plausible to DO, even if it is pacing.
Stage plays are unlike the movies. In a movie, the camera can focus on one actor doing something (even giving a speech) and the other actors are offscreen and don't have to do a thing. So the offscreen actors can be mannequins, and in truth may not even have been present while this scene was shot! This is not true on a stage, once an actor comes in, the audience sees them until they exit. So they must be exited once they no longer contribute to the scene or would be distracting. Until then, they need some work to do if they are going to stick around for another interaction.
That's why Mom hears a chime offstage and leaves to check on the bread for five minutes before she returns: We have no plausible busy work for her to do while Bob and Janet argue about his sister moving into the guest room, so we invent some errand for her to do offstage.
Get your head out of the page, act things out. Make sure things "flow" and "feel right" to you. It will change your writing, and to producers and actors is more likely to "feel right" too.
Let us use an analogy first:
Almost everyone goes through a phase in their teens where they write poetry. Agents and editors can give witness to the amount of poetry that is being submitted to them. At the same time, poetry is the one genre that sells the least number of copies. Because almost no one reads poems. This implies that most aspiring writers who write and submit poetry do not even read poetry themselves. According to aforementions agents and editors, this becomes clear from the lack of understanding of lyric principles apparent in the submitted poems.
Conclusions from the analogy:
If you want to (learn how to) write a stage play, read stage plays. What you read, will answer your question.
If you want your play to be performed, write with a particular theatre or acting troupe in mind, or a particular type of venue/ensemble. Research the sort of plays that they actually put on and think about duration, number of characters, technical complexity etc. as well as style and subject matter.
Try to involve potential performers or venues as early as possible in your writing process, even if it's just approaching a theatre or director an asking "Would you be interested in a play about werewolves that lasts six hours and involves a lot of pyrotechnics?" Many theatres these days have writing workshops or writing schemes you might be able to get involved with.
Get actors to read through your script as part of your writing process, both individually and as a group exercise.
As a writer, it's worth bearing in mind that a lot of theatre these days is "devised" to some extent, i.e. made up by the performers in a workshopping environment, often with a specific performance space in mind, so your idea of a writer's role in creating a play may be different from theirs.
There are two writing challenges that are especially sharp for playwriting: Suspension of disbelief, and understanding what is happening onstage. Accordingly, Aristotle recommended "three unities" for plays: One main action/plot, one location, one time frame. Although relatively few plays stick strictly to these rules, there are some good reasons behind them. You don't have the cinematic visuals in a play that make changing settings and timeframes compelling in a movie. And you don't have the time in a play to explain or play out all the subplots of a novel. So, the closer you stick to the three unities, the easier your play will be to produce, to understand, and to suspend disbelief for.
But what if you don't want to stick to the three unities? Death of a Salesman covers a lot of ground, but it's all mostly in Willie Loman's mind and memories. The "real world" stuff is all in the one location, within a relatively brief span of time. Man of La Mancha has a particularly clever solution, where the entire show is a play within a play. The frame story all takes place in realtime in a jail cell, while the play-within-a-play doesn't respect the three unities at all. That allows the audience to suspend disbelief and embrace the rudimentary set and visual effects.
In general I've found that the more successful realist plays I've seen stick pretty closely to the three unities. The ones that (successfully) discard the three unities are usually more memory-based or imaginative/fantastical. In either case, you want to limit your overall number of main characters and plots: if you have too many, they'll either get short shrift or make your play too long.