Real-Life Relative: Kevin's younger cousin Fuller is Macaulay Culkin's little brother Kieran.
Throw It In / Enforced Method Acting: Catherine O'Hara forgot her lines in the airplane scene where she realized that they left Kevin Home Alone. In fact, the iconic shouting of Kevin's name? Her attempt to salvage the take.
The conversation with the checkout lady and Kevin in the first was improvised. The director didn't say 'Cut', so the actress kept asking him questions and Culkin went along with it.
And if Home Alone 4 was successful? We would've had a TV series about it on ABC Family.
Originally Kevin's parents didn't get back together. The ending was rewritten in case they ended up making the show.
In the commentary for the first film, Chris Columbus said that he had planned for Home Alone 3 to be filmed right after Home Alone 2, which would've starred Macaulay Culkin again (Though it is unknown if there would be any difference between the original idea of Home Alone 3 and the one starring Alex D. Linz). He had also pitched an idea of another Home Alone in which Kevin McCallister, now older, escapes from jail and Harry and Marv have since then gone through a Heel-Face Turn.
According to AMC's Story Notes, Joe Pesci once said that his idea for a 3rd film would be Kevin lost in Hawaii, or a beach resort.
Daniel Stern refused to reprise his role as Marv in the fourth film, saying it was "An insult, total garbage." Day-um.