If comedies exist to make us laugh, “Horrible Bosses” is entirely effective. It delivers steady laughs beginning to end, escalates when it needs to and restrains itself enough to not lose the audience, a la most Will Ferrell movies.

But despite a cast with excellent rapport (and Charlie Day of “It’s Always Sunny in Philadelphia” at his lovingly madcap best), it doesn’t click on all of the levels it could have. It borrows heavily from “The Hangover,” but never seems to realize that if it took a few more risks, it would have just as much potential to be an enduring classic.

“Horrible Bosses” manages to stay fresh despite a not entirely original plot. Three friends hate their respective bosses and decide their lives would be better if the bosses were dead. What starts as hypothetical drunken banter turns into a trip to a seedy bar to track down a hitman. Like “The Hangover,” much of the comedy comes out of surprise and escalation, so the rest is left unexplained.

A principal joke in the movie is how similarly the characters’ situation parallels Alfred Hitchcock’s  “Strangers on a Train.” Like in that movie, the characters decide to “trade murders” in order to erase any motive in case they are caught.

Unfortunately, this awareness of influences is part of what keeps the movie from excelling. The problem isn’t that the movie is too close to “Strangers on a Train,” “Office Space” or “The Hangover.” Nothing is ever completely original, and those influences are balanced quite well here.

But the best comedies know when to add a jolt of originality, a quotable catchphrase or a bombshell twist. There’s a feeling of disappointment even behind the big laughs in “Horrible Bosses” that it’s not quite going far enough or treading its own territory. The pop culture references are better suited for a weekly show like “30 Rock” than for a feature film.

Kevin Spacey was born to play this kind of soulless slime, and Jennifer Aniston and Colin Farrell give their all as the other horrible bosses. They’re surprisingly effective villains, even beyond the over-the-top cameo factor.

The three good guys (well, aspiring murderers, but more on that later) have a fantastic rapport, but never quite define themselves enough to constitute an entire movie. Jason Bateman and Jason Sudeikis’s characters have no family or life outside their jobs to speak of, and Charlie Day’s fiancee only factors into two quick scenes.

This isn’t helped by the fact that Bateman and Sudeikis are much too similar and exist mainly to bounce witty dialogue back and forth. It’s genuinely funny dialogue, mind you, but it’s not tied to any three-dimensional character you can get to know. You’ll laugh, but you won’t remember them after you leave the theater.

Not only are the characters flat, but they’re glaringly inconsistent. The movie’s charm comes largely from the three main characters being likable, mostly rational, nice guys. Sudeikis’s character claims that people will die under his boss’s new environmental plan, but he’s the only one even close to an ethical argument for murder, and it still doesn’t fit with anything else his character has said or done.

I don’t care if comedic protagonists are moral or not. Comedy is specifically meant to push those buttons. But comedies do still have an obligation to make me believe in the characters, even if it’s on a ridiculous and offensive level.

Because of this, the ending has no choice but to be an obligatory, empty wrap-up. There’s just no way to resolve things where there’s no real substance to begin with.

That said, “Horrible Bosses” is the funniest movie to come out in quite a while. While its characters aren’t as strong, it nailed dialogue and big laughs much better than “The Hangover Part II.”

But even though “The Hangover Part II” disappointed me, it will likely stick with me longer than “Horrible Bosses.” The “Hangover” crew simply knows how to create an event movie, one with solid, lasting personas. Obviously, not every comedy with “Hangover” similarities has to work within the same archetype, but “Horrible Bosses” clearly sets up some unmet expectations.

Day’s energy alone is worth a ticket, and you’ll likely laugh more than at any other movie this year. I just wouldn’t expect a “Horrible Bosses Part II”.

Grade: B+