When I say I have a very vivid imagination, I mean this:
When I start thinking about how a scene or an interaction, or even something as small as a single reaction, might play out I can see it as if it's happening in front of me. Not exactly like a movie, more like as if I were watching some friends or family members from a couple feet away. If the "camera" is moving throughout the scene or if two characters are talking and it switches between them, that part plays out like a movie or tv show scene.
I hope this makes sense!