I went to see the movie Black Panther this morning. At night, my friend asked me about the movie. Should I say "The movie was great." or "The movie is great."?
You could use either. In practice, you would probably match the tense of the question.
Friend: Did you enjoy yourself last night?
You: Yes, I saw Black Panther, and it was great.
Friend: What do you think about Black Panther
You: It's a great movie.
If you are talking about the movie say "It is great". If you are talking about your night out say "It was great".