Dawson Casting: Jake Gyllenhaal was 23 when he was cast as a teenager for this movie; so was co-star Austin Nichols (J.D.). Arjay Smith, who played Brian, was 20. Only Emmy Rossum, who was 16, was actually high school aged.
Deleted Scene: Enough to make a second movie. Three main story lines were cut and one of them included a cameo by Alan Ruck.
Another one of such is an alternate version of the scene involving Taka the Japanese businessman killed in the hailstorm: instead of speaking with his wife on his cellphone, he's speaking with a man named Gary (later seen offering the bus driver $200 to let him and his colleagues on), who puts the shade in shady businessman, as they discuss the whistle about to be blown on a ponzi scheme of theirs, and what they can do to cover their asses.
Another is an alternate version where Jack and Jason continue to seek refuge in the Wendy's restaurant, even cooking themselves some hamburgers, and talking about mankind being capable of surviving ice ages, but needing to learn from their mistakes. Jack even tells Jason his version of the Greenland "vacation" he had with Sam, whom the later told to Laura earlier in the movie.
Too Soon: Roland Emmerich started writing the screenplay while he was filming The Patriot (2000). He stopped after 9/11, thinking a disaster movie inappropriate after such a cataclysmic real-life event.