Eliza Dushku spoke in an interview about starting out as Faith:

“I had done a bunch of films, and then wanted to get good grades to get into college, so I took time off to finish high school my junior and senior years,” the young actress says on one of her first days off since the show went on its seasonal hiatus. “When I got the role on Buffy, I was only supposed to come on for five to ten episodes. This project came up a month after I graduated, and we were thinking it would be a great stepping-stone back in.
“…I think they mainly just wanted to bring in this complete contrast opposite of Buffy, just to shake things up a little bit for the beginning of the season, and as we started doing it, the crew just started seeing some potential to make her a more involved character and keep her on. About halfway through, they told me we’re going to plan on making her into the villain… so I happily agreed, because it’s pretty fun playing the bad guy!
“…[Faith was] this bad seed and no one really knew what was going to happen with her. We all kind of worked together in creating her weaknesses, how far she goes, and the consequences. It was really gradual, it happened day by day. Mainly we made her turn into this girl in denial; she has more depth than just being the bad guy because she sheds light onto her background and why she is how she is. It’s not just one-sided, one single facet.”