Scott Patterson, who plays Luke, was not hired initially as a series regular. He was only signed on for the pilot episode. Although it was only after discovering the undeniable chemistry between him and Lauren Graham that he was contracted for more episodes, and quickly became a series regular.
Keiko Agena (Lane Kim) was 26 playing a 15-year-old in Season 1.
Liza Weil originally tested for the role of Rory and did not get the part, but the producers liked her so much that they wrote her the role of Paris Geller.
Alexis Bledel hates coffee, but Rory, her character, loves it. Whenever you see her drinking "coffee" in the series, she drinks Coca-Cola out of her cup.
Milo Ventimiglia and Alexis Bledel (Jess and Rory) actually dated in real life for three and a half years.