Update 10/23/14: This is an earlier version of this post, which I like a little better.
Video game genres: First Person Shooter, Real Time Strategy, Rail Shooter
Movie genres: Comedy, Drama, Horror, Fantasy, War
Genres in video games are named for how we influence the medium. Genres in movies are named for how the medium influences us.
A movie can have an intricate plot and still be boring. A movie can have a great character and waste his or her potential on generic interactions. Plot and character are elements of a story. The end goal is to entertain. This is a weirdly controversial point. We can argue the semantics… entertain could mean engage with us, take us other places, whatever. But the end goal is the same – a story is successful if it does these things and unsuccessful if it does not.
Entertain is a loaded word. People hear “entertain” and they think disposable popcorn thriller, comic book schlock, or some other ghettoized notion. But all movies entertain, be they a Michael Bay sequel, Masterpiece Theater, or an art house movie. The target audience may differ, the means by which it’s entertaining may differ, but they all entertain in some way.
That’s where genre comes in. Genre suggests how a movie will entertain.
Musicals are the most obvious form of genre because the characters are either singing or they’re not. Let’s say the average Broadway musical runs about 2 hours, has 20 songs, and those songs run 4 minutes long. That means that they spend 80 minutes entertaining us with song and dance, 40 minutes entertaining us by other means. If you like song and dance, you might be interested in a musical. If you’re not, you know to stay away.
Big deal, you might say. I don’t write musicals. Here’s why that matters:
Hypothetical example: Writer Alan Smithee pitches me a movie with a lot of world building and setup which requires a 30 page first act setting up a world of zombies, vampires, and an original race called the Organelle. He sets up the complex politics between them and threatens them with a war.
The script sucks. It’s all setup, no punch, the classic shitty second act .
ME: “There’s nothing in the middle. You need more action setpieces to prove to me why it’s necessary to learn all the setup you want me to learn. What’s the payoff?”
HIM: “Why does everything have to be about war or violence? Why can’t this be a story about characters interacting, talking. I want to write a drama?”
ME: Let me ask you this – why not write it as a musical? I know that sounds ridiculous, but seriously, why not?
HIM: Because I can’t write songs.
ME: Well, can you write dramatic scenes?
Genre suggests how you’ll be entertained. If you’re pitching me a drama, you’re pitching something with few unrealistic moments, where the entertainment value comes from watching characters have conversations. That’s really hard to do. Any idiot can fill a page with dialogue. If you’re writing a spec, you’re saying that you can write 100 pages of riveting dialogue scenes, where the words have incredible depth and meaning and the emotions behind them are as intricately plotted as an elaborate heist movie. That’s really hard. Compare a bad one hour drama to a great one. They both will cost about the same, they’ll both have staffs of highly skilled writers, but not all drama is great.
If you’re selling a drama, you’re sellling your ability to write amazing character based scenes. Not everyone can do this well. And if you can’t, it’s like you’re writing a musical with shitty songs.
According to IMDB these are the genres that exist:
ACTION, ADVENTURE, ANIMATION, BIOGRAPHY, COMEDY, CRIME, DRAMA, FAMILY, FANTASY, FILM NOIR, HISTORY, HORROR, MUSICAL, MYSTERY, ROMANCE, SCIFI, SPORT, THRILLER, WAR, WESTERN
I’m going to spilt them into two categories.
- Genres that suggest the emotional effect they create in an audience: ACTION, ADVENTURE, COMEDY, DRAMA, FAMILY, HORROR, MYSTERY, ROMANCE, THRILLER
- Genres that don’t: ANIMATION, BIOGRAPHY, CRIME, FANTASY, FILM NOIR, HISTORY, MUSICAL, SCIFI, SPORT, WAR, WESTERN
Category one is pretty simple. Action creates visceral spectacle, adventure takes us on a journey, comedy makes us laugh, drama illustrates human nature, family reassures, horror scares, mystery puzzles, romance is romantic, thrillers thrill.
Category 2 is more complicated. Animation and musical are styles of storytelling. You can achieve any of other genres through them, but musicals use music and animation uses animation.
Biography promises us the story of a person who actually lived. That person is going to be a simplified version (seecharacter 101 ) but they’ll probably need another elements genre to accomplish entertainment.
The others are settings. They suggest the world a story will take place in, but they are not complete genres unto themselves.
THIS IS THE POINT THAT TENDS TO GET ME IN TROUBLE
If there are no “pure” Sci-Fi films then what are 2001, The Matrix, Close Encounters, The Time Machine, War of the Worlds? If genre suggests how a film entertains us then what are the above films entertaining us with if not with Sci-Fi elements like Space Travel, A.I. concept, Alternate reality concepts, Alien life concepts, technological advancement concepts? Sci-Fi isn’t merely a setting, it is a device used to express abstract thoughts /questions like is time something you can travel through? Reddit user calprosper 
While sci-fi concepts certainly suggest an angle of exploration, you can’t solely promise a sci-fi movie and fully communicate how you’re going to make that idea entertaining.
Let’s say we’re exploring the butterfly effect. Wouldn’t you want to know if that will be explored via cool scenes of killing dinosaurs, mopey scenes with Ashton Kutcher, or brainbending tech talk like in Primer (which could be argued as a pure sci fi movie, but I see it as an indie drama).
Reddit user Supernovaploy  explains it thusly:
To understand, I think it might help to think of “science fiction” the same way you think of “fiction”: neither label gives the reader any indication how the story is going to progress. The Notebook and And Then There Were None and The Fundamentalist are all “fiction,” but you’d be hard-pressed to find someone who says they’re all the same genre.
Similarly (using Heinlein), Stranger in a Strange Land, Starship Troopers, and The Number of the Beast are all “science fiction,” but no one who’s read them will say they’re the same kind of story. Stranger is more political/romance, Troopers is political action/thriller, and Beast is adventure/(romance).
[In this paradigm,] “Genre” is an identifier that necessarily indicates a story’s construction, but not necessarily its theme. Because [cynicallad is] talking about story construction and not reading or watching for enjoyment, he’s breaking “story” into component parts. I do, for instance, think of “science fiction” as a genre – when I’m reading for enjoyment, I can take almost any sub-genre of sci-fi and have fun in the story, from Honor Harrington novels to Existence to Otherland. But when it comes to making sure all of a story’s required parts are present, thinking of “science fiction” as a genre doesn’t help me construct a solid story, so I shift my thinking just a few degrees.
THE CATEGORY TWO GENRES ARE MORE ABOUT SPECIFICS
Let’s say we’re writing comedies. We know that we want to create laughter in the audience. If it was a straight up comedy, we’d be setting it in our ordinary world, which requires little explanation. If we’re using something from the second category, we’re promising that there’s a damn good reason to use it, that much, if not most of the comedy will come from those specifics:
A biographical comedy might tell the story of a funny person (MAN ON THE MOON) or be a spoof send up of biopic tropes (WALK THE LINE)
A crime comedy will put a funny spin on crime specifics (ANALYZE THIS, SMALL TIME CROOKS)
A fantasy or sci-fi comedy will put a comic slant on a familiar genre ideas (YOUR HIGHNESS, SLEEPER)
Same with historical comedy, sport comedy, war comedy, western comedy (LIFE OF BRIAN, SEMI-PRO, STRIPES, BLAZING SADDLES). You get the idea.
There are dozens of genres (dieselpunk, alternate history, dogma films, nouveau vague, all those hyper-specific Netflix categories) but most every movie will lean heavily on at least one of the emotional primary colors suggested by the genres in category one.
No movie is purely one genre. They’ll all have moments of comic relief, or romantic interludes. An action movie is wise to slow things down with a dramatic scene. A comedy movie might have a genuinely thriller moment to ground the stakes in some kind of reality, but at the end of the day there’s usually one or two overriding genres, and that’s how movies are marketed, bought, and understood (sidenote: genres are like cats or bumper stickers. The more you have, the crazier you look).
You can mix them, you can subvert them, but genres exist and are a useful tool.
Premise is a promise that you can make an idea entertaining. Genre is how you entertain. If you understand how to use genre and premise, you’ll have a big head start when it comes to planning a script and filling in the sequences you’ll need to cover the 60 page death valley in the center of the script that we call the second act.