i personally like to make a sort of outline for the entire story first (or at least how i think the plots will connect together) and start working from there! i separate it in 3 different sections (first second third act) and think about how i want the characters to speak, if theyre going to rough and condescending or quiet yet mischievious ill try to express that in my script.
i also think about their motivations and what purpose they serve in my story so i dont make the mistake of adding too many useless characters,,, sticking with a script may be hard, but you just have to trust the process and itll turn out great! hopefully this helped a bit,,