Robert Pattinson rides rings around himself in 19th Century Paris, while becoming involved in the political tomfoolery. That and a series of longing looks is what holds this uneven, forced drama together. The British heartthrob actually made this film a couple of a years ago, but it's only now seeing the light of day - never a good sign.
Pattinson is a working class Frenchman, who through a somewhat of a convenient coincidence, ends up involved with the upper end of the social hierarchy with some plush Parisians. Said Parisians are generally women, and all look at him like he's a steak. Using this to his advantage, he sets about seducing them all; mainly to get in with their husbands (apparently that worked back then), but also because he just likes having relations an awful lot.
I can see why Pattinson was attracted to this role, and it's quite probably because it's so different than anything his tween idol status has thrust him into before. But Bel Ami is basically a mess, and other than the "provocative" scenes of him and a series of attractive, mostly older women, there's very little here for him to do. The political backdrop is never explored in enough depth to give the plot anything other than a glossing of supposed context.
Pattinson has been much better in superior films, and has obviously grown as an actor since this movie went into production. But he's hamstrung by a plot that replies far too heavily on his brooding looks to succeed. Thurman obviously had a hankering to get back into a corset after having huge success with Dangerous Liaisons a couple of decades ago, while Thomas is far too classy for the whole thing. Ricci, playing the only remotely redeemable character, is the only plus performance wise.
Unlikeable characters acting in generally despicable ways has made for interesting cinema before, but this lacks the wit required to make said scenes work.