What Sarah brings to the part is her intelligence, at the same time, she's got that hormonal, idiosyncratic goofiness that makes Buffy not just the Terminator.
Joss WhedonPassion... it lies in all of us. Sleeping, waiting, and though unwanted, unbidden, it will stir, open its jaws, and howl. It speaks to us, guides us... passion rules us all. And we obey. What other choice do we have?
Joss Whedon