When the screenplay isn't quite cutting it, you can always fall back on the natural improv talents of actors. John Candy, who played Gus Polinski, improvised some of his best lines in the movie. One of the movie's funniest moments of dialogue comes when Gus describes how he accidentally left his son at a funeral parlor for a day. That 100% Candy. (Source)

Kevin must've been really affected by Buzz denying him that cheese pizza. The actor who played Kevin—Macaulay Culkin—now plays in an indie band that performs parody versions of Velvet Underground songs…all themed around pizza. For instance "Sunday Morning" becomes "Pizza Morning." Yup. (Source)

Joe Pesci used some serious mind games against Macaulay Culkin on set—he tried to avoid him in order to make Macaulay think he was actually a bad guy. Also, he supposedly bit Culkin's finger for real while they were rehearsing the scene where Harry threatens to bite Kevin's fingers off. That's getting way into the role—some serious "Method" acting. (Source)

Sometimes even child actors (at least, those who go on to play in pizza themed bands) can actually improve a scene through the magic of improv comedy. Macaulay Culkin performed his famous scream differently that it was written: instead of waving his hands around after putting on the aftershave, he keeps them stuck to his face, like (according to director Columbus) in Edvard Munch's painting "The Scream." It adds an element of primal terror—like he really knows that he's home alone. (Source)

There will be no violent sibling rivalry stories in this interview: the actor who played Buzz, Devin Ratray, said that he actually got along very will with his own older brother and didn't use those experiences to inform his role as Kevin's older brother. He simply acted. (Source)

John Hughes was total softy. In the scene where Kevin sees a picture of Buzz's girlfriend and says "Woof!" Hughes thought that would be too cruel to do this with an actual girl's photo. So they put the art director's son in makeup and used that picture instead. Who says Hollywood people are all totally evil? (Source)

Since computer generated imagery wasn't widely used back in 1990, they simulated the scene where Harry gets burned on the head with a blow torch by burning a mannequin on the head in his place and then superimposing Harry onto the image using reflective glass. It's relieving to know they didn't actually burn his skull—though it would've sent his acting cred through the roof. (Source)

Originally, the script kept crime in the family, adding a sinister undertone and confirming Kevin's suspicions about a certain family member. In his first screenplay for Home Alone, John Hughes had Uncle Frank turn out to be the criminal mastermind behind Harry and Marv. Actually, it's kind of disappointing that they didn't stick with that plot point. (Source)

The actors could enjoy a dip on set if they wanted (we're assuming). At least, the flooded neighbor's basement Kevin wades through was actually built using the swimming pool of New Trier High School in Illinois where part of Home Alone was shot. (Source)