To begin with, the casting is good in this Hepburn film, in the sense that Charles Boyer, O'Toole, and Hepburn all play very well, all the other characters play adequately, but the film is fundamentally about O'Toole and Hepburn's relationship. Ms. Jackson's review summarises the plot well, so I shan't go into detail about that, but, having seen a few Hepburn films, I would certainly say that this is one of my favourites.
The film is charming all round, and there are some scenes which are very cleverly done and impressive. Well directed by Wyler, a favourite director of Hepburn's, and one senses the light atmosphere on set radiates through the camera lens onto the screen. The relationship between O'Toole and Hepburn takes a somewhat sudden turn to amourousness, and it all happens rather fast, but this is characteristic of older films, and is part of the quality which makes films like this so light and romantic. If four and a half stars were possible, I would certainly grant them.
Also, if you are confused by the fact that this film has been tagged on Amazon with 'lesbianism', 'teachers', and 'gay interest', this is most likely a mistake by reviewers, of this film with another Hepburn filmThe Children's Hour [DVD] [1961
] which is about lesbianism and features two female teachers (Hepburn playing one of them)... 'How to Steal a Million' is however, simply a romantic comedy set in Paris, with no teachers, and no references to lesbianism... Don't worry, you are looking at the film correspondent with this review!