[Ed. - This post will totally spoil A Perfect Getaway for you. It's a fabulous flick and you should stop reading now if you haven't seen it it yet.]
Of course you'll see the second-act twist of A Perfect Getaway coming. The script tells you flat-out at least three times that it's on its way. And only one twist is really possible. Of the four potential couple-killing couples in the film, one is obviously a "red snapper," one is arrested mid-film, and one practically screams, "We're the killers! Over here! Us!" The killers therefore must be the sweet and dopey fourth couple. Duh.
Those are the rules of the murder mystery as laid out by Agatha Christie, who in her forward to Cards on the Table wrote that often finding a murderer is as simple as identifying the least likely suspect.
Except that the twist in A Perfect Getaway is actually two twists that are revealed at once. The first is that mild mannered Cliff and Cydney are the killers. Like I said, that one is a bit of a duh, and it's really a clever ending, not a proper twist.
The second and truer twist is that Cliff and Cydney aren't Cliff and Cydney at all, but a pair of meth-addicted sociopaths who have killed them and stolen their identities. This second turn of the screw is a lot more fun, because it demands you hit the rewind button. Didn't you see Cliff and Cydney in their wedding videos? (You didn't.) Didn't Cliff and Cydney share a fearful conversation in their tent? (Watch it again. Their lines still make perfect sense.) Wasn't Cliff terrified of the killers? (No, he was worried about getting caught.)
A Perfect Getaway is a funny, scary, well-crafted thriller, with terrific performances by charismatic actors, especially Timothy Olyphant, whose presence saved Scream 2, The Girl Next Door and even Go. And Cliff and Cydney are an interesting metaphor for what it's like to be a newlywed, as two different wholes suddenly find themselves merely halves of a new, bigger enterprise, and must learn to act, speak and function in new ways.
In her writing, Christie pointed out the fundamental problem with whodunits. Creating characters like these means limiting your cast, which necessarily makes it easier to solve the mystery. A Perfect Getaway accepts her challenge, asking, "The audience will figure out 'who,' so can we instead build a twist around 'why?'" The answer is hell yes.