It's interesting (and appropriate) that Alain Delon was cast as the grand seducteur. For this role, he gained weight and used make-up to look older and tired. Fabrice Luchini played his valet. The movie was based on a book by Arthur Schnitzler and I read in a review that in the book, the valet did not exist. With the addition of a valet, the life of old Casanova is not as lonely as in the book.
Casanova becomes ill because Marcolina refuses him (or perhaps he is ill because he stands all night in open air watching her window after she told him her lover was to come). There is a touching scene when she refuses his offer of marriage because "I abhor your physic. I find you old and boring. You are egoist, always talk about yourself. Sometimes, when you are talking, you have bad breath. You belong to the previous world." Perhaps for Casanova, this is the first time he is refused and hurt. It's not that he only wants a woman, because in the same house, Olivo's wife, is his ex-lover, has been waiting for him for 13 years and ready to elope with him if he asks. When Marcolina's lover, the handsome lieutenant Lorenzi offends him, he uses all his might to beat him in the game of cards - without cheating as usual - and from his winning, he persuades the lieutenant to lend him his cloak, so when he enters Marcolina's room, the girl would think that he is her lover.
Casanova cannot be proud of his latest conquest because he gets the girl by deception. The movie concludes with his return to Venice and this one is also not something he cannot be proud of, because he has spied his friends.