Why do movie trailers give so much away?

Still from the new Star Wars film Rogue One. Photo / Supplied

When I walked into "The Beguiled" a few weeks ago, I assumed a few things. I was pretty sure it would be good, as director Sofia Coppola had taken the best director prize at the Cannes film festival just days before.

Second, I was pretty sure Nicole Kidman and her group of Civil War-era students were going to cut off Colin Farrell's penis.

The former bit of information I knew from following entertainment news; I got the penis idea from the trailer.

It's my preference not to watch movie trailers, which always seem to give away too much, but part of my job is writing a weekly synopsis of two trailers. So now I have to know way more than I want to.

Films have to be promoted; studios don't trust audiences to buy an unknown cow, so they drench us with free milk.

One of the worst recent offenders was the trailer for "Rogue One" that revealed Darth Vader would appear. And now, with the upcoming "Spider-Man: Homecoming," I and the rest of the world know that Iron Man plays a pivotal role.

Even small things can rob the audience of the pleasure of watching a character be revealed: Thanks to the "Justice League" trailer, I know that Aquaman has a wry sense of humor and swigs liquor out of the bottle.

And with "The Beguiled," the trailer put the possibility of Bobbitt-izing so front and center that it's easy to assume that's what the movie is about. It's not, and suggesting that is unfair to the work.

My husband and I constantly laugh at the previews for AMC shows, like the ones at the close of "Mad Men" and "Better Call Saul" episodes.

They're so ambiguous that the most you get out of them is confirmation that the characters you saw this week might be in the show next week.

And we watch the show the next week not because we know what's going to happen, but precisely because we don't. I'm not saying movie marketing has to go that far, but neither does it have to represent - or misrepresent - an entire film.

Trailers got their name because they used to "trail" the main feature, so at least then you had the option to leave. And most audiences did, which is why the "coming soon" slot was moved to take up the first 20 minutes of the moviegoing experience, with no concern as to whether people have a baby sitter at home, which, hello, is not cheap.

But now teasers and trailers and teasers of trailers are everywhere - TV and Facebook and Twitter all want you to see "War for the Planet of the Apes," and they're not giving up until they wear you down.

It's possible to build an audience by trusting that people can make the choice about whether or not to see a movie based on hints, not on full synopses.

Trailers should be more burlesque and less striptease - coy and tantalizing, rather than showing everything the movie's got.

After all, there's nothing like a little mystery to really get people's interest.