Some background about the show's creation and casting:
Lloyd Braun, ABC Television Group chairman, liked J.J. Abrams' outline for a new show so much that he ordered the pilot made before the script was even written.
A number of parts were reshaped to match the actors they found in the mad rush to cast the show. For example, Jack's character, not the pilot, was originally slated to be killed by the unseen monster. Once they hired Matthew Fox, they decided to let Jack live and become a mainstay character.
Yunjin Kim auditioned for the part of Kate (which later went to Evangeline Lilly). She (Kim) got the part of Sun instead, because her audition gave J.J. the idea of having her and a newly added character, her husband (played by Daniel Dae Kim), speak only Korean as the show unfolded. Yunjin Kim didn't think much of the subservient wife character she was supposed to play, so J.J. let her in early on the secret we (but not her character's husband) now know about her character's past.
Finally, the part of Hurley was first envisioned as a redneck, but was changed to the lovable joking guy we have because that's how Jorge Garcia is in real life!