I've already answered this very question on Quora, so I'll just copy and paste!

The hardest thing for me about screenwriting is remembering that film is a visual medium, and that your job as a screenwriter is to deliver a story that will be best conveyed via a giant screen in a darkened theater. When you're working strictly with words on a page, it's easy to get bogged down in the minutiae of writing snappier dialogue and cleverer stage direction.

Seeing a movie, especially on the big screen, is always a great reminder of the need to think in these terms when I'm writing. When the giant studio logo disappears and the film is about to start, you're not anticipating clever similes or one-liners; you're anticipating great images. Even in a comedy like The Hangover, the humor comes more from the originality of the imagery on screen than the cleverness of the quips. You might not remember specific lines from the movie, but you sure as hell remember the tiger in the bathroom.

A good rule of thumb for me (and I know I'm not alone) is that a great movie will sweep me up too much to really analyze it in the moment. Only afterward will I be able to recognize the nuts and bolts of what worked. And even then, great work often means it's hard to see the machinery. Some movies are just touched by the gods, and everything works so well that you can't tell where the writing, directing, acting, editing, etc. separate or overlap. That's one of several reasons why really good movies should be re-watched.

Badly written movies, I hear clunky lines and think, "Oof, that's why I bully myself about not writing such expository dialogue" or something to that effect.

I still watch the same amount. Only thing that really has changed is that I'm constantly saying, "I would have written that sequence different". I get flooded with thoughts on how I could write it and even shoot the scene differently. ('differently' is a lighter term than 'better'. hehe)

Things that always jump out at me now are; amounts of voice-overs, and the way exposition is handled. I've always had plot analysis apart of my viewing.

But until I get a script doctoring gig, I'll just continue to sit back and take notes on what works and what doesn't.

I notice structure more than I used to.

Heavy-handed exposition stings me.

And voice-over! Nine times out of ten, it's a weak-ass trick.

I agree with Amos. A good movie will overwhelm me. If I find myself analyzing the movie instead of being swept up in it, then it's
not a good movie.

My standards have risen, ironic, as now seems like a bad time for a movie viewer to have standards.

It's a bit like playing golf.

The mechanics of the game can ruin a great round if you think about it too much.

@Courtney: Have you ever used this trick on the golf course? A guy's about to tee off and you say, "Hey, do you breathe in or out when you swing?" Guaranteed to hit that drive deep into the trees.

Frankly it has taken me years to get beyond my analytical self when watching movies. The best I can do is a bifurcated mind: I have part of my brain tracking the 'guts' of the movie (i.e., structure, characters, subplots, setups & payoffs), the other part enjoying the movie for itself.

My best movie experiences nowadays are when I find myself reveling in both aspects of my consciousness: the movie itself and the underlying stuff. When I can enjoy both as the movie plays out, I'm in heaven.

But it's a very, very rare movie that so consumes me I can shut off my analytical self. And honestly I don't think I can even remember the last one. I have just come to accept the fact my analytical mind is part and parcel of my movie-watching experience.

I often feel frustrated and irritated by the obvious flaws in storytelling, directing etc. and I think what would I have done differently as a writer to make it work better. If watching at home I swear, curse, shout and make my girlfriend crazy with my comments. But sometimes there´s absolutely nothing wrong with the movie and I sit still, hardly blinking, and just inhale the magic and afterwards wonder what the hell hit me. Then I can start slowly analyzing what brought out the magic and see if I can touch it.

Analytic. Definitely. But, also can easily get lost if the story is really working. I (almost) always think, "there's a reason they showed that _____" and am waiting for the payoffs...

I keep sweatin' the budget on every detail. I otherwise struggle to keep my mouth shut out of consideration for others in the room who don't want to hear about deneauments, no third act, "ooops didn't foreshadow that one" and crap like that. shhh Think it don't say it.
Frootcayke5150 (Health & Safety Code...look it up) aka @LilPeep2u on Twitter.

@linda: Good to put a name to a Twitter handle! I hear this a lot from writers: how insufferable we are around others when watching a movie at home, with friends on DVD. I remember once screening a movie for possible distribution rights back when I was at Trailblazer Studios, calling out every single things that was going to happen before it happened. Less a testament to my understanding of the craft than the by the book nature of the movie in question.

I'm guessing it's a great to way lose friends and lovers. So your advice is sage: Keep mouth shut!