Joe Pesci deliberately avoided Macaulay Culkin on-set because he wanted Culkin to think he was mean.
Everyone on-set was amazed with how mature and professional Macaulay Culkin was. Joe Pesci told Entertainment Weekly, "Mac is not like a nine-year-old. He's an old man already."
Joe Pesci and Daniel Stern felt indifferent about the movie's potential during shooting, so they intentionally gave over-the-top performances, neither one of them believing this movie would become a massive success.
Although John Hughes was fiercely defensive of his screenplay and insisted that everyone deliver his lines as written, he allowed his friend John Candy to improvise. This was actually a trademark of writer and producer Hughes, who also had the five students in Clube dos Cinco (1985) improvise when they told one another why they were in detention. In the film, almost all of Candy's dialogue is improvised.