During Miles' interrogation of Uncle Buck (22:36 to 23:01), John Candy spoke the script's dialogue, so Macaulay Culkin could say the lines back to him and keep the pace of the scene very fast. (Note that this would be the take of the scene when the camera was set for close-ups of Miles.)
Despite their characters not getting along for the majority of the film, off-camera Jean Louisa Kelly got along very well with John Candy and said she was honored to work with someone she considered a legend.
One night during filming, John Candy went to a bar with Music Supervisor Tarquin Gotch, and spent most of the night there meeting people. The next day, John Hughes heard a caller on a radio talk show excitedly describe his evening with Candy. Hughes was upset with Candy, and despite Candy's assertion that Buck was supposed to appear disheveled, Hughes cancelled his scenes for the day, and told him to get himself together and get some sleep.
Almost every set was built in a local high school gymnasium, including the two-story Russell house.
The noise that Buck's car makes when it backfires is that of a gun shot and a firecracker.