Trivia: In the scene where Alex approaches the two girls at the record stand you can see the original score for 2001: A Space Odyssey, also directed by Stanley Kubrick.
Continuity mistake: Look at the scene where Alex beats Georgie and Dim and they fall in the river. Alex politely offers his hand to Dim, and in the background you can see Georgie with no hat. Shortly afterwards, he's got his hat on.
Continuity mistake: When Alex is informed that he's being transferred to the Ludovico center, the governor's arms jump from his lap to under his chin, then back to his lap. (01:06:00)
Chosen answer: It's part of the "fun element" of the crime to get the victim to open the door themselves.
Captain Defenestrator