When MD first came out, I craved understanding.

I sat down one evening and spent eight hours in a single viewing (though far from my first viewing), watching virtually every frame. By the time I had finished, all the questions that came with the DVD were answered. (Though I never understood why "Irene" at the beginning was blind.)

I'd make the claim that the key to understanding is knowing when/where the actual beginning is. (Something to do with The Cowboy, and far into the movie.)

And "the rule of three" is also very important. Some of that is clarified only when certain fast-moving frames are frozen. Also, though perhaps more obvious, is the color blue.

And the killing is not of ONE person, it's two. (Listen carefully to the wording.) And I believe that scene to also be the "key" to the blue key.