As the raffish filmmaker Coles Burroughs, Mark Ruffalo again proves himself one of the most appealing actors on the screen, even when playing a thoroughly obnoxious character. Coles has quite a history, dating back to his Sarah Lawrence College days when he was involved in a wild sexual triangle with Sam (Maya Stange) and Thea (Kathleen Robertson). It all ended messily, but ten years later in Manhattan, Coles, now successful in advertising and married to Claire (Petra Wright), runs into Sam, recently returned from England. They are also reunited with Thea, also married and calmed down considerably. That great first love between Coles and Sam still smolders, however, and is re-ignited, with devastating results.

First-time writer-director Austin Chick shot XX/XY for a pittance, but his film is nonetheless rich in ideas and emotions. Coles, whose mantra would seem to be his spoken "There's no room for honesty in a healthy relationship," is an unfortunate, all-too-common type of non-confrontational yet never-satisfied male, waffling even when his wife begs him to tell her the truth. Adultery comes a lot easier to him than honesty of any kind and, were it not for his killer attractiveness, he would probably be the total loner he instinctively wishes he were. Ruffalo really makes the film work, and it's hard to imagine anyone else who could combine such qualities of innate scruffiness (even when cleaned up and yuppified), guarded mystery (concealing a big zero, actually), and easy sensuality. When an irate young moviegoer demands that Coles refund the money he wasted at Coles' one film, Ruffalo's underplayed reactions are a superbly complex mix of bemused superiority, a glimmer of dawning awareness and rueful humor. (He forks it over.)

Chick authentically renders the frenetically hormonal, drink and drug-fueled post-adolescence days, when the bravura notion of falling into a mnage à trois can seem as natural as anything. (The director also captures the impossibility of sustaining these complex deals, and how brutal the end can be.) He's strong, too, in his portrait of a marriage, in a nicely shot, mirrored scene of Coles and Claire having a suggestive bedtime conversation, but in the bathroom, while scrupulously brushing and flossing their teeth. The strongest-written role is actually that of Claire, and she has a great final denunciation of her wandering fool of a husband, in which she reads him for the big coward he is. Wright tears into her speech with an avidity that must have been as satisfying for the actress as it is for the audience.

Unfortunately, Stange is too unsure an actress to inspire much interest or believability as anyone's Big No. 1 Love. (Admittedly, sometimes in life, it actually does work out this way, in terms of that person being actually somewhat less than all that.) Robertson has fun with her sprightly role as a determined winner in life, no matter what. She is paired with David Thornton, who brings the same bizarrely imitative Garry Shandling quality to this as he did to the recent off-Broadway play String Fever.