Preparing a scene does not mean inventing as much backstory as possible. It means understanding the script well enough to make specific, playable choices.
Step 1: identify the circumstances
Where are the characters, what has happened, what do they know, and what is the immediate situation? Mark script facts separately from imagination.
Step 2: name the objective
What are you trying to achieve from the other person? Write an action sentence. If the answer is only a feeling, ask what the character wants to make happen.
Step 3: define the obstacle
What makes the objective difficult? The other character may resist, circumstances may be urgent, or the character may be avoiding a truth.
Step 4: choose tactics
How do you try to change the situation? A character may reassure, distract, challenge, bargain, or confess. Tactics can shift when information changes.
Step 5: locate the change
Identify what is different by the end: new information, a changed relationship, a decision, a refusal, or a revised plan. Let the scene develop.
Try it in your next rehearsal
Use this method on a page or less of dialogue. Write one sentence for each step, perform the scene, then change one tactic and observe how the scene shifts.