I read a very insightful article recently about what makes a 'good' television actor, and one of the key things mentioned was the ability of the actor to give their character an 'internal world', as in- how they act when their not acting.
How they convey emotion and intent when they don't have any lines/directions in the script. Do they have believable reactions to other things around them.
I think that's one of the things that struck me about Faith as a character (especially in my recent viewing of S3)- how we the viewer know so little about her but we get a sense of that internal world of hers, even if its just through the inflection of a line of dialogue or the way she looks at a certain person.
It adds a richness to her character that is, quite frankly, some very impressive acting for a 17 year old Eliza who was really only brought in for a throwaway role.
The fact that she just keeps on improving every time she's brought back is just a nice bonus. 🙂