Its Jazzy music, running through the film, sets the tone for quick witted dialogue, full of turns, conversations that shouldn't have been heard, but meant a lot, and basically, dramas from couples which nevertheless love each other. As Vincent, obviously the character the director mostly likes, says near the end: "all important stories are love stories. But that doesn't mean they are all 'easy going'" I'm a fan of Karin Viard. She is constantly on the edge during the trip, but seems to learn something from the experience. She's very expressive, any lesser actress wouldn't be convincing at her, yes, hysterical moodiness. She's beautiful, but the film builds on that image a bit too much to my taste.
François Cluzet's Marc is a great counterbalance to all this "feeble humanists". He's mean, self centered, and incapable of empathy. I liked his "successful" character, specially after seeing him doing "Antoine", the exact opposite "a childish depressive and broke 'crosswords writer' for a local magazine" with the best G. Dépardieu at Les Apprentis (1995).
Rose-Marie and Caroline are fine, but I would have liked a bit more of "character development" for both.
Overall, I loved this movie, felt really well in the end (not the usual case for French films :)) and would definitely watch it again to get the subtle dialogue, the "lapsus" (Anne always "breaking a bottle") and the sheer enjoyment of two lovable characters who couldn't be more difficult. The tenderness scene at the fireplace while stranded in the middle of nowhere has a refinement rarely seen these days where sex scenes are used when there's nothing to say.
Shot at Lyon, and the city is referred at -mostly by Anne- about 8 times.