People probably have different philosophies about this, but I think that when you're first shaping the play and trying to find a character, the initial actors that develop it end up imprinting on it - you hear their voices; you hear their rhythms. You can't help but to begin to write toward them during the rehearsal process.
In senior year at college, Paula Vogel was my playwriting teacher; she is the first person to introduce me to the notion that a woman could actually forge a career in the theatre. Up until then, the possibility seemed remote and inaccessible, as I had very few role models who directly touched my life.