I remember reading that they have an algorithm that picks the "best" (colorful, cinematic, attention grabbing...) picture to show in the UI from the content itself. That is why sometimes the poster used represents a very minor character instead of the main lead.

Not only that, they dynamically change the poster to try to make it more appealing to each person and it still fails miserably.

They wrote a blog post (that I can't seem to find) a few years ago about how they A/B test their thumbnails.

Similarly, I'm not sure if I like that they play a short scene of a movie when you hover vs. showing the original trailer. Sometimes I like to watch familiar favorites but they just show me one short scene and it doesn't really draw me in as I had hoped.

You can turn that off, but you have to go to Netflix settings on a web browser and turn it off for each profile. Because reasons.