Remove all the prepositional phrases. In this sentence “about real people” is a prepositional phrase. “About” is the preposition, “people” is the object of the preposition, and “real” modifies “people”.

Now you’re left with “Movies interest me.” You should be able to take it from here.

When in doubt, removing prepositional phrases from a sentence will nearly always reveal the true subject. What is confusing you is that the object of a preposition is a noun, and when there are a lot of nouns in a sentence they all tend to look like a subject. Find the prepositions and kill them off. The subject and verb will survive. Trust me.

Eliminating the extraneous words, you could say “Movies interest me.” Which movies? The ones about real people.

It would not be the same to say “People interest me.”

And by the way, it’s “grammar.”