Write What You Know: It's believed that Daisy was based off of Genevra King, Fitzgerald's ex-girlfriend who was also a rich Chicago socialite. Likewise, Tom is believed to have been based off of William Mitchell, the man who Genevra eventually married. Fitzgerald himself confirmed that Jordan was based off of Edith Cummings, one of Genevra's friends.
The 2013 film:
Actor Allusion: Nick reaches for the last drink on a tray too late and it's snatched by someone else. That exact thing happened to Peter Parker in Spider-Man 2.
Dawson Casting: Carey Mulligan (26) as as the 23 year old Daisy and Leonardo DiCaprio (36-37 during filming) as the 32 year old Jay Gatsby, who also play those characters in flashback when they are 18 and 27, respectively.
36 year old Tobey Maguire as 29 (later 30) year old Nick Carraway.
Genius Bonus: The bandleader at Gatsby's parties is named Trimalchio, which is a shout-out to The Decameron where a character by the same name is famous for his elaborate parties. Additionally "Trimalchio" and "Trimalchio in West Egg" were possible titles for the book.
At the end of the "'Can't repeat the past'? Of course you can!" scene, Gatsby walks between a statue of two ravens which could represent Oden's ravens Huginn (thought) and Muninn (memory) — appropriate since Gatsby's thoughts are always on his memories of Daisy.