Reddit users discovered that John Candy, who played "polka king" Gus Polinski, was the only actor screenwriter and producer John Hughes allowed to improvise.
John Candy: 23 Hours and Pure Improv
According to user SundayRed, Candy showed up on set for just one day and shot all his scenes in 23 straight hours. His paycheck was symbolic — just $414, since he did it as a personal favor to Hughes.Candy's comedic talent wasn't accidental: he trained at Toronto's Second City improv school (1973 graduate) before appearing on Second City Television.
For "Home Alone," he literally "invented" his character on the spot, and Hughes left all the dialogue unchanged.

Source:
imdb.com
Other Fun Film Details from Reddit Users
- The "gangster movies" with the line "Keep the change, ya filthy animal" never existed. They're specially shot 80-second fake clips made for the first and second films.
- If you listen closely during the paint can scene, you can hear Harry's gold tooth hitting the floor.
- Kevin's mom is into sewing: her house has mannequins and a sewing machine. These details explain where the boy gets his trap ideas and why his mom looks so put-together throughout the movie.
- Harry (Joe Pesci) wears a wedding ring. In the sequel, it's gone — fans joke that his wife didn't forgive the jail time.
- Elderly neighbor Marley has a bandage on his hand throughout the film. But when he reconciles with his son in the finale, it disappears — a symbol that he's "healed."