Answer:
Suppose you were the director of the movie: how would you change this scene?
Step-by-step explanation:
The correct verb form to fill in the blank would be the past tense were.
For a sentence to be grammatically correct, it must follow the rules of concord which is basically subject-verb agreement which means that if the sentence is in singular form, the verb should also be in singular, if it is in past tense, the verb should also be in past tense, etc.
From the sentence, the modal auxiliary verb 'would' is past tense, so therefore, 'were' is correct to use because it is also in past tense and they are in agreement.