Ability over Appearance: When casting Julia, producers wanted a relatively young actress (the character was 15 at the time) but ended up casting the 19-year-old Neve Campbell due to her strong attitude during the audition.
Dawson Casting: 19-year-old Neve Campbell and 26-year-old Scott Wolf played 15-year-old Julia and 16-year-old Bailey respectively. See Ability over Appearance to note that producers tried to avert this with regards to Julia.
Fake Brit: Alison the gay English girl is played by Aussie Poppy Montgomery.
A lot of other main cast members who would have been unknown back then - such as Scott Grimes (Will) who would later star in ER, Band of Brothers and Ridley Scott's Robin Hood. Ditto for Lacey Chabert (Claudia) who is now more known for Mean Girls or her voice on The Wild Thornberrys. And of course, Matthew Fox became Jack Shepard.
Star-Making Role: For Neve Campbell, Jennifer Love Hewitt, Matthew Fox, and Lacey Chabert.