The actress appears as unfulfilled wife Clotilde de Marelle who falls for 19th century Parisienne womaniser Georges Duroy, played by Robert, and admitted she was won over by his charm in real life.
She told the Daily Mirror newspaper: "He`s a sweet guy and super funny. And a really good kisser.
"He`s so charming and smart, we would just goof off and laugh while we were working."
When asked what her favourite things about the movie were, she added: "The costumes, the make-up and Rob".
Christina, 32, admitted that even though Robert - who is dating his `Twilight` co-star Kristen Stewart - has only starred in handful of films compared to her 22-year Hollywood career, he was able to give her some helpful advice about acting.
She added: "He`s so organised and in charge of his career. He was actually giving me advice!"
Christina is not the only `Bel Ami` actress to have been impressed by the 25-year-old actor, as Uma Thurman has praised his talents in front of the camera.
Uma - who plays another of Georges` married lovers Madeleine Forestier - said: "He`s very serious. He did a huge amount of rehearsals in his own time. I think that`s what you do when you`re a young actor, when you take your work very seriously and want to take it to the next level."