The trailer for the video game Assassin's Creed: Unity starts with a bird's eye view over a packed city square in Paris, 1789. A revolution is at hand here: A poverty-stricken boy sobs as his mother is beaten by a castle guard; a chambermaid fights off the sexual advances of a nobleman. As a low-voiced monologue intones incitement to revolution, a mysterious league of assassins emerges from the shadows to slice the throats of royal guards, leap en masse from rooftops, burst through lace-curtained windows, swing on ropes through high glass windows into an ornate ballroom, and overturn a white-and-gold carriage with a noblewoman inside. "A tidal wave of change," promises the monologue.

You'd never guess from the gorgeous animation of the trailer that the game itself would end up a glitchy mess that could hardly be played without crashing. Characters sometimes showed up without a face, or with their legs on their heads; the protagonist had a troubling tendency to defy gravity and fall upward, into the sky, or to fall through the floor into a featureless abyss. Ubisoft had to release a patch a month later to fix the all the bugs left in the code.

Beyond this, the Assassin's Creed: Unity trailer didn't feature any footage from the video game. None of those locations appears, at least not in much detail or scope. And what of the chambermaid smashing the guy in the face? The mother and her sobbing son? Those characters never show up in the game.

This is business as usual for the games industry. Because games tend to be in development right up to release, trailers have to be made long before the coding is completed. Joseph Kosinski, who directed the spot, didn't work with the game's software at all. "For me it's not important that the game is done," he said, "because I'm just using that as a starting point to kind of create a customized world that tells the story in the trailer. Personally I find the concept art to be usually more inspiring than the games themselves."

Kosinski has made feature films such as 2010's Tron: Legacy, but he is also a prolific director of game trailers, the most famous being an award-winning trailer for Gears of War, featuring Gary Jules's haunting cover of Mad World. That was one of the rare trailers made using the game's engine—the code that renders a game's animation and movement—but none of the imagery shown appeared in the final product. Nothing that occurs in the game's so-called Mad World trailer actually shows up in the game itself, not even in a cut scene or a flashback.

"Typically, what I like to do is take the world of the game and interpret it a bit," said Kosinski. "So you're taking inspiration from the concept art for the game, and maybe some beta—you know, if they have a version of the game that partially works—and based on that, we'll build our own version. For me, it's not important that the game is done, because I'm just using that as a starting point."

Working with a movie director like Kosinski might be a go-to choice for many high-profile game companies such as Epic Games and Ubisoft, but it's not how every big-budget game studio operates. Blizzard Entertainment, best known as the company behind World of Warcraft, released a team-based multiplayer shooter this past spring called Overwatch. The fervor around Overwatch attends not only the game but also its accompanying trailers and promotional videos, all created in-house by Blizzard's own animation team.

Since the game is ongoing, its creators face the unique challenge of having to constantly maintain players' interest. The Overwatch team manages that by releasing new in-game features every now and then and by releasing an animated short every month or so. All the Overwatch promotional shorts take place within the game's setting, depicting its characters—who also happen to look incredible.

"We're trying to create a universe, not necessarily just an advertisement for this game," said Jeff Chamberlain, the VFX supervisor of Overwatch.

Chamberlain hopes these shorts will serve not merely as trailers to convince outsiders to buy Overwatch, but also as encouragement for current players to learn more about the rest of the world of the game. "My secret dream is that maybe, if you get into Overwatch because all you want to do is—you're interested in action, you want to show your first-person shooter skills off—that maybe you hear a line of dialog in the game, or you see something visually interesting, and you're like: 'You know what? I want to know more about that Hanzo and Genji, seems like they have some history.' And you check out one of the movies, and you get introduced to the story and the animated shorts that way."

Tumblr and Twitter are teeming with fan-art of Overwatch characters, featuring references and backstories that can be found only in the animated shorts. The gameplay of Overwatch isn't narrative-focused at all, but fans don't seem to mind because the promotional videos serve up plenty of canonical story. Comments sections for the animated shorts are filled with fans asking Blizzard to make an animated film about the Overwatch characters, or to develop a full-length narrative campaign for the game.

If Overwatch's realistic trailers draw fans in, though, unrealistic ones can set them off: After a much-anticipated game called No Man's Sky debuted this past August, gamers were enraged. Early trailers and demos had promised an expansive solar system packed with unique, computer-generated planets, each one teeming with unfamiliar life and encouraging endless exploration. In practice? The game felt purposeless, packed with menus and chores. Angry players took to Reddit, posting exhaustive lists of features they'd been "promised" that never materialized.

It led one games critic to question whether fans were too complacent about the gulf between trailers and the games they advertise. "We all remember BioShock Infinite's fake trailers, which seemed like "gameplay" but were really only thinly-veiled first-person cinematics, none of which ended up in the final version," Brendan Caldwell wrote on the site "Just because every game developer under the quintillion suns does the same thing, does not make it OK. The right question to ask is: Why do we think this is an acceptable thing within our industry? Why are we prepared to buy into an intergalactic spectacle and then shrug off the discrepancies when that spectacle turns out to be only spectacle?"

Despite this plaintive cry, however, gamers generally accept that most trailers don't have much to do with how a game will turn out.

The YouTube comments on Kosinski's AC: Unity commercial, for example, are filled with adoring praise–for the trailer, not necessarily the game. One user wrote, "let be honest guys, coming from someone who has played every single AC game….Ubisoft does amazing CG trailers, but the actual games always come short." Another user agreed, "This trailer gave me to much hype." A third wrote, "the trailers was awesome … but the finished game was only crap."

It's not as though game companies don't still release "gameplay" footage to promote their games as well, not to mention playable demos so prospective buyers can give the game a shot before they buy it. But cinematic trailers still seem to hold an emotional sway—enough to generate way more viewership than any "gameplay" trailers.

The promotional gameplay videos on the official YouTube channel for Assassin's Creed tend to have hundreds of thousands of views, but the "cinematic" trailers tend to have over 1 million views (in the case of Kosinski's trailer for Unity) or even 8 million views (in the case of the Assassin's Creed: Syndicate cinematic trailer). This could be due to repeat viewings from fans who love the look and feel of the trailer and want to experience the fantasy version of the game over and over again.

Cinematic trailers may not genuinely represent what the game will look like, but they aim to represent what the game will feel like. It's paradoxical that Kosinski has become one of the go-to masters of this balancing act, since he calls himself an "outsider" to gaming.

"Personally," he said, "I don't play games at all."