In the weeks leading up to the release of "The Vow," it was easy to knock the movie: the amnesiac plot line (Rachel McAdams' character wakes up from a coma and must be wooed anew by hubby Channing Tatum), the lovelorn glances, the schmaltzy sentiments.  Come to think of it, it was easy to knock the movie after it came out too; self-knowing irony isn't exactly the name of this game, which lends itself to all sorts of comic opportunities from the cheap seats. That Tatum and McAdams' acting, which in recent years has been characterized by his stoned-faced qualities and her chipper ones, hasn't lately made the Oscar voters come running added to the fun.  Yet after the weekend's heart-stopping box office — $41.7 million, well above expectations and in fact the sixth-highest February opening in history — it's clear that, for all the ways one might compare this movie to a cross between "50 First Dates" and "While You Were Sleeping," we still rushed out to see it.PHOTOS: 'The Vow' premiere  The truth is it shouldn't be entirely surprising. McAdams and Tatum are shaky leading draws in movies that aren't romances — see under "Morning Glory" and "The Eagle." But they do OK when star-crossed love enters the picture (see under: McAdams' "The Time Traveler's Wife" and "The Notebook" and Tatum's "Dear John.")  They're apparently even more persuasive when they're star-crossed together: "The Vow" is on pace to take in more money than any of those films, and in fact more money than any movie than either of them has done as leading actors on their own, save for Tatum's "G.I. Joe," a different beast entirely.  How does that work exactly? Why do actors we're only lukewarm on apart work when together? Certainly the traits that can seem like too much on their own — say, McAdams' perkiness and Tatum's earnestness —can be complementary when mixed, two extremes somehow neutralized, the filmic equivalent of sweet-and-sour sauce. It's why Meg Ryan's constant poutiness and Billy Crytal's relentless wise-guy-ness worked well in "When Harry Met Sally" (also, incidentally, a better performer than many movies they did on their own), or how Audrey Hepburn's effusiveness and George Peppard's stoicism made for a classic in "Breakfast at Tiffany's."  Not to compare this movie to those classics. But in that sense, at least, "The Vow" has located the formula of many cinematic romances — they work best not necessarily because the actors seem like a real-life couple, but because the traits of one half mitigate the other.  Review: 'The Vow' leaves you wanting more  'The Vow' leads strong weekend with $41.7 million  'The Vow' fails to live up to its promise, critics say  Photo: Rachel McAdams and Channing Tatum in "The Vow." Credit: Screen Gems
The Vow:  Channing Tatum and Rachel McAdams draw huge crowds for the opening weekend of their new romance "The Vow."