More of an exercise in narrative gamesmanship than an actual thriller, A Perfect Getaway pretty much douses its first half’s methodical build-up of suspense with its second half’s bucket of contrivance. That’s not to say it isn’t a lot of fun — it is, with a sly sense of humor and sharp dialogue that makes clever, reflexive reference to the characters’ presence in a comic whodunit. (“He’s really hard to kill,” declares one, doting lovingly on her boyfriend, who may or may not be half of a couples serial-killing team.)