When a performance feels rehearsed in the wrong way, the actor may have decided what to do before the other person has finished affecting the scene. A convincing reaction begins with attention: what did you hear, what did you notice, and what does it change?
Listening has an objective
A character is rarely listening in an abstract way. They may want reassurance, search for a lie, wait for permission, or hope the other person reveals something. Choose what matters to the character, then allow the words to provide new information.
Do not manufacture a reaction
A pause, widened eyes, or sudden breath can be useful when it comes from the situation. Repeating the same reaction every time information arrives makes the performance predictable. Stay available to the scene instead of decorating it.
Let new information change tactics
If your objective is to persuade someone to stay and their response is anger, reassurance may fail. You might become direct, ask a question, or reveal what you were avoiding. A reaction is meaningful when it changes what you try next.
Review the cause, not just the look
After a take, ask what you heard and how it altered your action. If you cannot identify the information that caused a reaction, test the scene again with a clearer listening task.
Try it in your next rehearsal
Perform a short exchange once by concentrating on your planned lines. Repeat while focusing on what the other character wants from you. On the third attempt, choose one piece of information that would change your tactic. Write down one observable difference.