Choosing an activity for a scene is a very practical matter.
Think about your real life. It’s full of activity, and all of it is practical on some level.
You go to work, because you want to get paid, and you do whatever you need to do to get the job done that day. You eat because you’re hungry or because you have a dinner date with someone. You read the newspaper because you want to be informed. You go shopping because a lightbulb burned out and you need a new one. You pick up the mail because you haven’t been to the post office in a week, because you have to buy stamps anyway so you can pay your electric bill, or because you’re waiting for a package and you hope it came in today.
What do these things have in common? The word “because.”
In other words, you always have a reason for anything you do.
Your characters are driven to do things on stage for the same reason. Their lives are not governed strictly by the dramatic events of the play. The rest of their lives continues unabated, just as it does in ours. If someone close to you is hospitalized, the grass doesn’t stop growing, the dogs don’t stop needing to take walks so they can pee, and the refrigerator doesn’t refill itself on its own.
Much of the activity that should be taking place on stage is NOT written in the script. If it bears directly on the events of the play, it will. For instance, if your character’s company is treating its employees unfairly and the employees decide to strike, your character may be making picket signs in the next scene, and the dialogue might refer to that. The dialogue might not refer to it, but you might choose to make signs as your activity anyway, because it makes sense in the context of the play. But if you chose to make dinner during the scene, that might work just fine, too.
Whatever you choose as your activity for a scene, it must make sense to the audience. This doesn’t mean it can’t be unusual or unexpected. But if your hardworking banker husband comes home from work and, without ditching his suit, starts to do ballet warmups using the back of the couch as a barre – that’s an unusual choice that the script better justify on some level. If it seems entirely uncharacteristic, given what the playwright has written and how the actor chooses to play the role before and after working the barre, then a different choice that the audience will accept is in order.