Through a series of questionable decisions I have written and directed four plays. In that time I learned a small amount about how to write good plays, and far, far more about how to write bad plays. And now I'm going to tell you how.
Trap The Audience And Gradually Crush Their Spirit
I truly believe that nothing is terrible in quite the same way as a terrible play, mostly because the audience isn't allowed to leave. If you get bored of this article you can just close the tab, and I never need to know. Not so in the theatre.
This isn't helpful advice, it's a warning. If you write a play it's your responsibility to make sure that play doesn't chip away at the audience's soul. To that end, I think it's helpful if your guiding principle in writing is not "how do I write a play" but rather "how do I keep a captive audience entertained".
To that end, never ever do anything because you think it's the kind of thing that should happen in a play. Do it because it's entertaining for the audience. Crucially, I'm not saying you should be going out of your way to do the sorts of things that don't normally happen in plays, I'm saying that the question of what ought to happen in a play is entirely secondary to the question of what an actual audience will be glad they watched. Make it worth watching.
Don't Worry About Space
As a general rule, whenever something famously difficult to do well seems suspiciously easy to you, it is probably because you have failed to notice the hard bit.
Case in point, it sure seems handy that in a play you can just do dialogue. Not like a book, where you have to worry about describing the setting or actually conveying the delivery of the lines. Why, in a play, it's a good thing if it's not clear from the script whether or not somebody is shouting, because then the actor and the director get to feel important.
Beware, though. In a book the positioning is free. If somebody stops talking, the reader can more or less forget they exist. In a play, the actors are just standing there. If they don't have anything to do, they'll keep standing there.
Have you ever watched two people have a passionate conversation while standing perfectly still? It's horrible. It's up there with two people having a conversation while randomly pacing around.
Good plays are dynamic. The characters move as they speak, and interact with the stage around them. A good director can help a lot here, but dear lord, don't make it too hard for them. Give your characters reasons to move around, things to pick up and put down.
Similarly, don't just leave people on stage! If they're not saying anything, they should be either doing something or leaving. If they linger as some kind of weird pseudo-audience member, they will distract the actual audience members.
Trust The Audience To Pay Attention
Plays are not like TV and they are not like books. You don't get recaps, you can't flick back a few pages, and you can't google a character to work out who they are. You can't even really ask the person sitting next to you.
If you want to have a clever twist, by all means do it: drop your little hints and weave your little web. But remember that your actors are going to stumble over their lines, your audience is occasionally going to get a bit distracted, and also it's actually pretty hard to remember a bunch of people's names. So when you pull off your crazy twist, if the audience has forgotten all your clever little hints, they'll just be even more confused.
If the audience is meant to know something then make sure they've been told it a good three or four times, loudly and clearly.
Well, I think that'll do it for writing. Subject to my own whims I hope to eventually vent about the horrors of directing and producing as well. All things in good time. If you are for some reason planning to put on a play I'd be delighted to hear from you, and I promise to be slightly less pessimistic.