Screenwriting can be painfully slow

Ideas... It all starts with ideas. A free flow of thought containing untold possibilities of adventure for characters yet to be thought out to their fullest extent. Next the logic centres of the brain kick in and suddenly the shackles are on. One high concept idea stays stuck on the mind, then the story begins to unfold. Beat by beat, each step of the journey forms into one coherent adventure... But the scenes are yet to be made. The story is there, the characters, locations and drama all ready to be sliced into the first draft. It's at this point that writer's block steps in. Now staring at a white page, all the tabs set for the action and dialogue but no words will come out. Every thought feeling just that little too on the nose to be worth writing in...
My advice to any aspiring writer is simple... Finish the script!
Don't worry about what you put down or about what works and what doesn't. Don't panic if you started with pet dogs in one scene and then later decide cats would be better. Do not go back to fix anything until you have finished that elusive first draft. Let yourself flow until the story is over. Leave that first draft in the drawer for a week if you have to, because its only after the first draft is done that you will be ready to fix everything you feel is broken with it.