For the Oxford scenes, real Oxford students were recruited to play their 1920's counterparts. For the scene where Charles and Sebastian first meet, real members of the exclusive Bullingdon Club can be seen behind him wearing their traditional blazers. A few of them can also be seen in the luncheon scene that takes place the next day.
After Charles "betrays" Sebastian in Venice, there is a shot of an altarpiece in an Italian church, with a depiction of the "Martyrdom of St. Sebastian." You can identify him by the arrows protruding from his body.