During Miles' interrogation of Uncle Buck (22:36 to 23:01), John Candy wrote out the script's dialogue, and wore it atop his head so Macaulay Culkin could read the lines more quickly, 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.)
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 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.
The movie was originally intended to be shot in the St. Louis area, rather than John Hughes' traditional Chicago. Filming was about to begin, when the decision was made to move the shoot to Chicago (the reason being, that the film was set in winter, and the winter of 1988 was unseasonably warm in St. Louis, leading to the change).
John Goodman was considered for the role of Uncle Buck, he would later go on to play Fred Flintstone in The Flintstones (1994), a role, for which John Candy was also up, as The Flintstones (1960) was his favorite cartoon, sadly his premature death in 1994 prevented this.
Bill Murray, James Belushi, and Dudley Moore were considered for the role of Uncle Buck. Bill Murray and John Candy starred in Stripes (1981). Candy and James Belushi starred in Only the Lonely (1991).
When Buck goes to the party to find Tia, he hears rap music playing ("Bust a Move" by Young MC), and jokingly says "Who's that? The Grass Roots?" The Grass Roots were a popular band in the late 1960s, that had a string of hits, including "Lets Live For Today", "Midnight Confessions", and "Temptation Eyes", amongst others.
Uncle Buck asks Miles, who is interrogating him, "What's your record for consecutive questions asked?" Miles replied, "38". John Candy was thirty-eight-years-old at the time this movie was being filmed.