Read the instructions, read the entire scene, identify the facts, decide what you need from the other person, and prepare one specific way to pursue it. Then rehearse enough that you can respond, not just recite.
Start with the material around your dialogue. What happened before the scene? What changes during it? What does the other character say or do that should affect you? Mark the moments where your approach needs to shift. For a self-tape, test the frame, sound, light, eyeline, and reader before your best take. Follow the requested file and slate instructions exactly. Preparation is not deciding every facial expression in advance. It is giving yourself enough structure that pressure does not erase the story. Arrive with a choice, then stay available for a different one if direction comes.