But having a rough idea as to how your story starts, progresses and ends will certainly help when it comes to putting words on the page. This roadmap can be any length, long, short, fat or thin. And as detailed as you want it.
Throw in some dialogue for good measure. Get some of those characters interacting and talking to each other. Pick a scene, and get writing.
There's no single way to write a screenplay, but the more work you do beforehand will certainly help when it comes to the actual writing.