When directors cast actors, they often work with a casting director. Casting directors are people who know acting talent: they comb theaters, workshops, and acting schools, as well as watch movies of all kinds looking for new talent. Directors must to their casting director for as long as it takes it order to impart to them the essence of the characters as they imagine them. Avoid speaking about physical characteristics, because they limit the scope of the search. The author may write the role as someone who is short and dark in appearance, and a tall strawberry blonde actor may walk in who could be better for the part because he has the essence you seek for the character. But also recognize that in some instances it is impossible to avoid speaking about physical characteristics if it makes it easier to explain your vision.