Decades ago, Hollywood used to put previews of their coming attractions after the conclusion of their theatrical releases. The teasers earned the nickname “trailers” because they followed the feature film.

Today, trailers aren’t such an afterthought. Studios spend millions of dollars stirring up anticipation for their big-budget movies by releasing trailers that promise consumers something worth the hassle and expense of a ticket. The responsibility for taking the most dazzling 120-odd seconds from hours of footage and splicing it into a coherent—and compelling—mini-movie falls on trailer editors, who screen films months in advance in order to create previews that will build the viral buzz filmmakers look for.

To better understand the job, mental_floss spoke with several editors at three of the most highly respected firms in the business. Here’s how they get you excited about the next blockbuster.

1. YOU NEED TOP-LEVEL SECURITY CLEARANCE.

If you think studios are worried about rough cuts of their films falling into the wrong hands, you’d be correct. As some of the few pairs of eyes outside of the production to see a movie months before release, trailer houses must make sure their offices can’t be tapped by potential pirates. Ron Beck, the owner and creative director of Tiny Hero, says that only employees at Fort Knox might be able to relate to the level of security that trailer editors deal with. “There are cameras everywhere,” he says. “We have sensors that record everyone who goes in and comes out of a door.” Rough cuts of movies typically get delivered on encrypted hard drives and are edited only on hardware that’s inaccessible to an open network.

“All of [the studios] are careful, but Marvel leads the pack,” Beck says. “Their stuff is super-strong. That’s why you rarely see their movies pirated.”

2. THEY MIGHT BE SEEING AN ENTIRELY DIFFERENT MOVIE THAN YOU DO.

In order to begin work on marketing campaigns, trailer firms are usually given extremely early footage that has yet to be polished and edited. Rough cuts might emphasize plot points or characters that wind up getting minimized by the time the picture is done, or “locked.” David Hughes of the UK-based firm Synchronicity says he’s seen a few movies that he barely recognized once they hit theaters. “Bridget Jones's Diary was quite dark at one point,” he says, “and I recall a totally different opening to Bowfinger where the film-within-the-film was called Star Wars rather than Chubby Rain because the accountant who wrote it was so stupid he didn't know a film called Star Wars actually existed."

Since films continue to get pared down right up until release, it’s also common to see scenes in trailers that don’t ultimately make the final cut. “Dirty Rotten Scoundrels [is] my favorite example, because someone wrote to complain that they had waited the whole film to see Steve Martin push an old lady into a swimming pool, as seen in the trailer, only to find that the scene wasn't in the finished film.”

3. THEY CAN ASK FOR SPECIAL EFFECTS TO GET PRIORITIZED.

Because editors see films so far in advance, they’re often looking at footage full of green screens and unfinished effects work. But if an editor feels like a scene would bolster the trailer’s impact, they can request the studio fast-track the CGI. “We can’t ask what they shoot first, because productions usually revolve around an actor’s schedule,” Beck says. “But we can ask for visual effects stuff we need to be done first.”

4. THEY MAY CUT A TRAILER YOU NEVER SEE.

Daniel Lee, who spent 10 years at Mark Woollen and Associates before migrating to the buzzed-about firm Project X, says that editors are often called upon by directors or producers to splice together a “sizzle reel” made out of stock or existing footage in order to sell a studio on a movie. “It’s becoming increasingly common to do,” he says. “It’s an inexpensive way to sell someone on the vibe of a movie.” Director Joe Carnahan commissioned a reel when he was looking to direct a theatrical version of Daredevil (above).

5. THEY DON’T LIKE SPOILERS ANY MORE THAN YOU DO.

For last summer’s Terminator: Genisys, fans who viewed the trailer were slightly annoyed to learn—spoiler—that perpetual victim John Connor was a Terminator in yet another revision of the franchise's confusing canon. But those edicts usually come down from the studio, according to Beck. “I like to tease, not tell,” he says. “In certain movies, though, you have to give it up, or the trailer won’t even be good. Revealing a twist is ultimately the studio’s decision, though.”

6. THE 2003 TEXAS CHAINSAW MASSACRE REMAKE REWROTE THE RULES.

Trailers are often the result of other trailers that studios noticed were particularly effective in engaging an audience emotionally. One example: the preview for 2003’s Texas Chainsaw Massacre remake. “The one that always comes to mind is the trailer for the Michael Bay-produced remake of The Texas Chainsaw Massacre, where black frames were inserted off the beat to disorienting effect,” Hughes says. “This technique has been borrowed for many horror trailers since, including some that we've made.”

Another trend-making trailer: the one for 2010's Inception, with its thunderous "braam" sounds that seemed to influence every heavy action/drama film that followed.

7. THEY CAN’T HAVE PEOPLE POINTING GUNS AT OTHER PEOPLE.

Because trailer content is subject to many of the same ratings restrictions as the feature film itself, editors often have to cut around some of the Motion Picture Association of America (MPAA) mandates. If a trailer is a “green band,” or suitable for general audiences, that means no threatening people with firearms. “There’s a lot of minutiae, like where a gun can be pointed,” Beck says. “You can’t have someone pointing it straight at the camera, for example, or at anyone in the same frame. Sometimes we blow up [zoom] a frame to hide stuff like that.”

8. TRAILERS GET FOCUS GROUPS.

Studios looking to reach the widest possible audience sometimes like to hedge their bets on campaigns and enlist two different trailer vendors to create edits for the project. They’ll focus-test each and back the one with the most support. That’s not unusual, but what irks editors, Lee says, is when a studio’s marketing department decides to split the difference and create a trailer based on ideas from two different creative entities. “They might combine trailers,” he says. “We call that Frankensteining.”

9. THEY CAN GET ACTORS TO SAY ANYTHING.

Because editors have precious little time to communicate the theme or premise of a movie, having a line or two of dialogue that summarizes a character’s motivation can make all the difference. Unfortunately, not all movies come stocked with exposition. If a trailer needs a clarifying line and the actor isn’t available to record dialogue, Beck can go in and splice together sentences from words he’s already said. “We might use a sound-alike actor, or we might see if we can form whatever sentence with the lines we have. We could make ‘I need to find her’ from someone saying ‘Find her’ and ‘Need to.’”

If all else fails and an actor is needed, Hughes says there’s one relatively quick fix. “If you've seen a film in the last five years, you've probably seen a film in which at least one line of ADR [Additional Dialogue Recording] was done on an iPhone after the actor had left the set.”

10. THEY LIKE TO LEAVE PRIVATE EASTER EGGS IN TRAILERS.

Studios love when fans of film franchises dissect trailers to spot hidden references or clues. So do editors, but sometimes the Easter eggs they drop in are going to be hard for anyone outside of their family to catch. “I know a few editors, myself included, who try to slip in their voice in a piece,” Lee says. “That’s only if you have enough time to fiddle with it.” Lee’s two kids lent their voices to a sound mix for World of Warcraft: Looking for Group, a documentary about the game. “I don’t know if they made the final cut, but they’re in there.”

11. COMEDIES ARE HARD.

Of all the film genres he’s overseen, Hughes believes comedies that don’t hit the mark are his worst assignment. “I've made trailers for comedies where there were literally not enough jokes in the film to fill a trailer,” he says. “Going back in the mists of time, I remember the trailer for Beverly Hills Cop III having one joke in it, Serge saying something sarcastic about Axel Foley's shoes, and then they cut that joke out of the film.”

12. SOMETIMES YOUTUBE AMATEURS CAN BREAK IN.

Fall down the YouTube rabbit hole and you’ll find thousands of movie trailers cobbled together by hobbyists outside of the industry. While many might underestimate the work and craft involved in doing it professionally, a few have been able to use it as a launching pad to get noticed. “I know one or two editors who got careers because of their YouTube channels, where they were uploading stuff completely as a hobby,” Lee says.

13. THEY LISTEN TO A LOT OF MUSIC—SOME OF IT UNRELEASED.

Beck believes the majority of a trailer’s impact can be chalked up to how the images fit with the music selection. “Music is at least 50 percent of any trailer,” he says. With access to unreleased tracks from music labels, Beck will go jogging with his earphones in to sample tunes, even though he might not find a perfect visual fit for a song for months. “I’ll picture a scene and maybe see something like it a year or so later. And then I’ll go, ‘Oh, I’ve got just the song for this.’”

14. RYAN GOSLING AND MORGAN FREEMAN ARE TRAILER GOLD.

Ever since voiceovers for trailers largely went out of style, editors have needed to keep viewers oriented in other ways. But that doesn’t mean they can’t cheat a little. Beck says that editing a trailer for anything containing Morgan Freeman is like having a narrator. “We did Now You See Me 2 recently, and when I knew we had Morgan Freeman in the movie, I knew the whole trailer was going to be driven by him saying his lines. He’s like the voice of God.”

Another go-to performer: Ryan Gosling. Why? “He just nails it,” Beck says. “He can convey a meaning or moment so quickly that you can use it in the trailer. You’re trying to do so much in a short amount of time, and when an actor is emotive, it makes my job easier.”