I've mentioned this before, but it bears repeating: when you write a script, either for stage or screen, you don't know which actor or actress will play the characters you create. So you have to focus on what they are inside, more than what they look like. You will know the basics of sex and age and you need to know what they do (and sometimes don't do!). But the bulk of your time is spent figuring out what they want, what they will do to get what they want and who will try to stop them. I have an article on my website about this process called "Getting the Wood Out." In the article I go into more detail about my character discovery process. I quote from that article: "In his book, Playwriting: The Structure of Action, Sam Smiley identifies basic character traits, from the simple-tangible to the complex-intangible, that are important to good character creation. By understanding and using these traits, you rise above simple craftsman to character magician, resulting in characters that linger after the last page is turned". Everyone has different methods of character discovery and I am a huge proponent of sticking with what works. The end goal is the same: creating characters with beating hearts that hook the reader and keep them from first word to last.