I've been working on scene design lately, and I am struck again with the essentiality of getting the events in the right order. The scene is building towards something-- an action, a revelation, a decision, a conversation, a punchline, something. And how you get there matters.
One thought -- the scene events should be caused by other scene events. Don't blow the sequence here.
So Protagonist is a teacher. She comes into her classroom and sees the mean, punitive principal standing over Mark, the student she's been hoping will overcome all the obstacles of his background and go to college.
Now imagine the principal immediately tells her that Mark was caught smoking weed on the football field. If instead you have the principal say, "I'm kicking him out," without explanation, then Teacher can bristle and protest, and the principal can yell at her for going against him, and it can get really close to her quitting as a protest against his treatment of her student, and THEN finally Mark can be driven to admit his crime—so that he can cut this short before she quits or gets fired.
Make sure you don't have the big reveal (the moment where we learn what Mark did) happen too early. Also understand that it matters who reveals it. Should the principal use the truth as a weapon? Or do you want Mark to acknowledge his own error? This depends on what your purpose is (and where in the book this scene is). But usually big reveals, disasters, and realizations happen near the end of the scene, for greater drama and plot propulsion.
A young couple is packing to go away for a weekend.
The phone rings. It's the husband's brother, who says he's coming to town with his irritable wife and four loud, disobedient children (well, the brother doesn't characterize them that way).
Husband says, well, we won't be here.
Hangs up. Wife says, isn't it fortunate that we were leaving town.
They leave town.
Now that's no fun. They planned something, and what they planned isn't changed by the events in the scene. It means that what happens in your scene doesn't matter.
Now how hard is it to make characters react? To make their actions reactions?
Let's try it:
Husband and wife are sitting around, contemplating a nice relaxing weekend at home.
Husband's brother says he's coming and bringing wife and kids.
Husband thinks fast and says that unfortunately, he and wife are just packing to go out of town-- and he gestures to wife to start packing so it won't be a lie. Oh, shoot, we won't be here to visit with you when you and the family come into town!
Now they have to go out of town, just in case bro comes by and knocks on the door.
So they pack quickly and leave town, as a result of what happened in the scene-- the phone call.
Scene design is all about sequence, about designing the events of the scene to create the most powerful effect. Cause-effect is not the only organizing principle, but in the western tradition, it is probably the most powerful.
Whenever you think, "And they just happen to ... (do something, plan something)," that "just happen to" is a signal that you are not thinking causally. I'd go so far as to say virtually nothing should "just happen to happen" in a story. It might SEEM that there's no cause, that it's random, but we are Westerners, and we don't actually think very much happens at random (at least not in fiction).
Just keep that in mind: You are in charge. You can design a scene. This is fiction. You can make the events happen in the way that leads to the most drama.
(Of course, if you are writing, I don't know, Waiting for Godot, you'll want to subvert this Western need for causality by making it seem as if something is going to happen, but nothing does. Story of my life. :)