The tune that the Marquis de Sade keeps humming throughout the film is the French children's song "Au Clair de la Lune" by Jean-Baptiste Lully, the second line of which becomes increasingly relevant - roughly translated, it is "lend me your quill so I can write a word".
The trivia items below may give away important plot points.
Though the events in the film are based on reality, they have definitely been 'tarted up' as the opening disclaimer says. In real life, the 'Marquis de Sade''s second wife, Marie-Constance Quesnet, was living with him at Charenton, along with their daughter. He was sleeping with Madeleine Leclerc (Kate Winslet's character) who was only 13. And contrary to Geoffrey Rush's physical appearance, the real Marquis was, by this point in his life, extremely obese. He never had his tongue cut out, and died in bed.