Okay, so your protagonist is a 17-year-old orphan and heroin addict living on the streets of Hollywood. She wants to be an actress, convinced that fame will solve all her problems, but what she really needs is just to be loved. Eventually, she ends up in front of the camera on a porn set somewhere in The Valley, and that’s when the real tragedy begins.
This is a good start. You have an interesting character, full of flaws (addiction, abandonment, self-absorption), she has a clear goal, gets locked into a complicated situation, and most importantly, you understand the screenplay’s theme. However, if this is all you know – just the tip of the iceberg – your script will fail.
As the writer, you must know everything: the entire iceberg. What was it like growing up as an orphan? When was she abandoned? Does she remember her mother? Her father? Was she a foster kid? How many families? What about school? Sports? Friends? When was the first time she had sex? Was she raped? How did it happen? Is it a secret? You get the idea.
Asking character questions is your greatest alley, because when you fully understand the entire life of the character prior to page one of the script, your work is already done. The character writes her own dialogue; she leads you through the script. And then, what happens is believable.