In the trailer for 1993’s Free Willy, a film about a boy trying to liberate a whale from captivity, audiences learn that—spoiler alert—the boy is able to free Willy. In the preview for 1994’s Speed, which features Keanu Reeves trying to disarm a bomb placed on a bus by madman Dennis Hopper, viewers see the passengers watching the bus detonate from afar. And in 2000’s Cast Away, possibly the most notorious preview in recent memory, we see that Tom Hanks—who spent much of the movie stranded on a desert island—is able to escape.

Movie trailers have certainly not gotten better about subtlety in recent years. In 2015’s Terminator: Genisys, franchise character and savior of humanity John Connor is—another spoiler alert—revealed to be a machine.

Why does Hollywood appear to be so hell-bent on removing any traces of mystery from film previews?

It turns out we only have ourselves to blame. Or, more specifically: focus groups.

Studios usually farm out trailers to companies specializing in editing and promotion. Dozens of different versions of a trailer will be edited and then shown to test audiences in order to see what they find most appealing. According to marketing executive David Singh, the response is usually that viewers like more—more action, more dramatic beats, and more spoilers.

"It’s such a competitive world out there," Singh told Marketplace in 2019. "You’re competing for people’s time with ... every platform imaginable. You’ve got to tell them enough to get them excited about it."

In editing these trailers, "big" moments often elicit the strongest audience responses and also serve to remind viewers the film being marketed is unlike anything they’ve seen before. When Singh worked on 2015’s The Martian, featuring Matt Damon as a stranded astronaut on Mars, marketing executives were faced with the fact that Damon had just played another astronaut in 2014’s Interstellar. To get audiences excited, they felt the need to lay out the new film’s ambitions—including the fact Damon likely does solve his predicament.

For Terminator: Genisys, director Alan Taylor said there was concern over making sure audiences saw this new Terminator as something different than the four Terminator films that preceded it. "I know there was kind of a challenging calculus going on in the heads of those who market this thing to decide that [revealing John Connor to be a machine] was the right thing to do," he told The Hollywood Reporter in 2015. "I think they felt like they had to send a strong message to a very wary audience that there was something new, that this was going to new territory. They were concerned that people were misperceiving this as kind of a reboot, and none of us wanted to reboot two perfect movies by James Cameron. I think they felt they had to do something game-changing in how the film was being perceived."

Taylor said he had "unpleasant conversations" about the trailer giving so much away. But some filmmakers embrace the more-is-more approach. Director Robert Zemeckis defended the trailer for Cast Away by saying audiences want movies spelled out for them. "We know from studying the marketing of movies, people really want to know exactly every thing that they are going to see before they go see the movie," he said. "It’s just one of those things. To me, being a movie lover and film student and a film scholar and a director, I don’t. What I relate it to is McDonald’s. The reason McDonald’s is a tremendous success is that you don’t have any surprises. You know exactly what it is going to taste like. Everybody knows the menu."

Despite criticism, the strategy does appear to work. Jason Blum, the producer behind Blumhouse and some of the most successful horror films of the past decade—including Paranormal Activity (2007), Get Out (2017), David Gordon Green's Halloween (2018), and The Invisible Man (2020)—has said that the technique is, like Michael Myers, a necessary evil.

"People don't like it but almost everyone that says they don't like it go[es] to see the movie," Blum told Insider in 2020. "The trailer really isn't for those people that are so tuned in, it's people who are kind of thinking they may or may not go. One of the ways to get them to go is to show them a lot of the movie in a trailer. And I thought we should have shown more of the movie in The Invisible Man trailer."