Why do you ask this question? I'm guessing the reason is know if it ought to be closed as off-topic according to this answer. But I don't think asking "Is X trivia?" is sufficient to decide if something is on or off-topic.
I also don't think that making trivia automatically off-topic is useful. And here's why...
What is trivia?
matters or things that are very unimportant, inconsequential, or nonessential; trifles; trivialities.
The very definition of "trivia" shows how completely subjective the term is. What is unimportant to you may be very important to me.
Is this question trivia?
To me, it is. I've never seen the film, and have no interest in seeing it, so everything about that film is, to me, very unimportant, inconsequential, or nonessential. However, to a person who enjoys that film, knowing how improvised it was may have great importance, and may enhance their enjoyment of the film. There are many films I do like that have improvised portions, and knowing that makes it more enjoyable to me.
Should this question be off-topic?
I do not think this question should be closed as off-topic. But if we do choose to close it as off-topic, it needs to be for a better reason than "because it's trivia." The vast majority of questions on this site are trivia to someone. In fact, one could say the entire topic of this site is trivia.
And before our FAQ says "trivia is off topic," I think we need a far more precise and objective definition of "trivia" than is provided by a standard dictionary. (In other words, I think we shouldn't make trivia, per se, off topic, but instead use some other objective standard to define which questions are "too simple".)