When we get a brief, we break it down to identify all the different pieces. We develop concepts for the way these are put together to achieve maximum impact. An overall concept is then scripted - and it's the script which operates as a blueprint for everything that follows.
Every production needs a solid script. It will contain information that the cast and crew need to do their jobs. Actors, directors, post-production, sound. Everybody needs to know how to take the story off the page and onto the screen. The narrative structure (and everything you need to know about the chronological turn of events) should be detailed within.
It must contain the fundamental elements: Where, when, what, who and how. But it must provide more than that. Scripts tell us about the characters. Informing us about their age, their disposition and their personality. Small details about personality can support casting and beyond this, get the best performance from your talent. Details about the environment can offer more than just the minimum requirement of 'where'; it's one thing to describe 'a dining room'... It's another to describe 'a large, open dining room with large windows, flooded with daylight'. Immediately, you have much more to work with.
Beyond the physical, details about behaviour, speech and gestures can also be greatly influential. A script should be rich with interesting dialogue and prompts to ensure actors give convincing and natural performances. It should offer indications of inflection, emotion and stage direction. So is 'He walks into the room' is a stage direction. 'He bursts through the door with a fierce expression on his face'. Which statement does more?
To make a truly compelling film, you need a professional script. One that details the important information, but more importantly, stimulates your creativity and gives you ideas about how to tell the story. It's the difference between the script for Home Alone and 'Boy get's left at home alone. Burglar's attempt to steal from the home. Fail. Family comes back. The end'.