When I was in high school, my drama department participated in yearly one-act show competitions. The concept was simple; take a show and perform it in front of a panel of judges to win trophies for your department.
The catch was that these shows needed to be less than 55 minutes long on the dot or you would be completely disqualified, and even the most simple one-act plays are usually designed to run an hour or longer. On top of that, my drama teacher liked for us to perform full-length plays with recognizable names. So, we spent a decent chunk of time cutting shows down for time.
What annoyed me was that often, when we did this, important parts of the story would be removed, and the remaining show wouldn't make any sense. Characters would appear that had their subplot removed, making their part unnecessary. Entire scenes would move so fast that the show felt rushed and choppy rather than like an actual show.
So how do you cut a play script without breaking the pacing and storyline? Here's my advice as a writer and as an actress.
Preserving the Story
The most important part of cutting a play script is preserving the story that's being told. In this instance, you're not a writer; you don't get to decide what the plot is and what the characters say. You're an editor - it's your job to help the plot advance in the most efficient and effective way possible.
If the cuts you're making completely change the show that your audience is about to see, you need to start marketing the performances as an "homage to" rather than an "adaptation of." There's nothing wrong with reinterpreting a show into something new, but there is something wrong with telling your audience they're going to see a show that they aren't.
That being said, if you're changing the story that much, you'll probably run into issues with your licensing to perform it. Remember, you're paying to use someone else's words; if you're changing those words drastically, you're breaking your contract. Check your performance agreement before you make any major cuts.
Subplot versus Duel Plot
What I think trips a lot of first-time script editors up is telling the difference between a subplot and a duel plot. It's a fine line, but an important distinction.
A subplot is a storyline that is happening in tandem with the main storyline but doesn't directly affect it. These are the supporting roles. The actions of subplot characters might be a part of the larger story, but they're generally going to be less impactful and serve more to flesh out the main plot than anything else.
A duel plot is a storyline that is happening in tandem with another storyline but carries just as much weight. Both stories are played out by main characters, carry the same thematic weight, and often impact each other in significant ways. We care about both plotlines equally.
Subplots are things like comic relief character shenanigans and some romances. Duel plots are characters telling the same story from another perspective or adversaries working to stop each other. A subplot can be removed without changing the story, but a duel plot can't.
Cutting Characters - A Cheat Sheet
Cutting individual characters is significantly easier than cutting entire storylines, but it still deeply affects the plot. If you're going to do it, do it right. You can cut characters that are:
Part of a subplot you're removing
Not directly tied to the main conflict
Playing a bit part (on stage for one or two lines as an introductory or transition character)
Easily combined with another, larger character
If the character you're removing would significantly affect the plot of the show, you should consider simply shortening their part rather than fully removing them. Cut them from non-key scenes but leave the most important parts of their story - driving action, connections with other characters, and serious life events (such as death) - intact.
Quality Cutting Makes Good Shows
Your show can be short and still good. I've seen tons of one-act shows that absolutely blew me away by keeping the story of the original and condensing it in a way that didn't feel rushed or cheap.
What's really going to sell it is your actors. They can't act as if they're working from a cut script - it'll be obvious which parts are missing. If you're acting in a cut show, pretend that the cut pieces don't exist, and tell the story that's in front of you. If something's not working or it doesn't make sense, the actors should be able to come to the director and tell them. Work it out together and make sure you're still telling a complete story.
Cutting a script for time is stressful, but not impossible. As long as you stick to the story, know your plot types, and respect your characters, you'll do just fine.
And...time!