This odd combination of story, characters, and cast could easily have fallen apart irretrievably in the first few minutes. That it holds together is due primarily to Audrey Hepburn's unsurpassed charm and Billy Wilder's resourceful story-telling technique. It ends up being enjoyable most of the time, sometimes very much so, in spite of itself.
The story is rather strange - for it to "work" you have to buy into a number of unlikely possibilities, and even then, you have to accept the main characters as sympathetic even when they don't deserve it. It's the kind of hollow concept that you see much more often in present-day movies, which are made for audiences who don't care about plausibility, and who are easily persuaded that a shallow, pseudo-romantic attraction between two characters automatically makes them sympathetic.
None of that is to imply anything against the stars. Audrey Hepburn is so engaging as Ariane that it makes you want her to be happy, even though much of her behavior is fatuous. Maurice Chevalier is enjoyable and is obviously well-cast, and John McGiver also adds some good moments. Gary Cooper's character doesn't work very well, but that should not be blamed at all on Cooper. The character just is not as appealing as the scriptwriters presume him to be, and Cooper should actually be commended for making him as likable (or as un-unlikable) as possible.
Wilder's skill made some strange stories work pretty well in his time, and he also deserves much of the credit for keeping this one afloat. There are also some very good sequences in the screenplay, for all that it was uneven in general. The odd thing about "Love in the Afternoon" is that if you can tolerate the poor setup and get past the obvious flaws, you can really enjoy most of the movie, because it does have several positive things to offer.
