Look how dour everyone is! Sure, upon closer inspection you can see that Kristen Bell* is making a funny face, but if you were to just glance at it, everyone looks really grim. And the tagline "It may be paradise... but it's no vacation" could easily be from a slasher movie rather than a comedy about couples who hate each other. (I guess? I seriously only have the barest inkling of what the movie is about.)
And unlike the image above, the ad near my house lists Vince Vaughn's name above the title, making it seem like he's the only actor in the movie anyone might have heard of. That guy's been in dramas! Maybe he had a house payment to make so he had to do this quicky thriller. Sure, you might notice Jon Favreau on a second glance, but he's been in dramas, too. It takes quite a bit of looking to recognize Jason Bateman and start to think that maybe this is some sort of comedy.
What I'm saying is that if you had not heard of this movie and only sort of paid attention to the ad, it's possible that it could look like a movie where some of these people are killed with a knife.
*I didn't know Kristen Bell was in this movie until I hit Google Image Search to find the billboard image. If Google is to be believed, she spends this entire movie in a bikini. This makes me want to see the movie 25% more than I did before. Unfortunately, 25% of zero is still zero.
