Writing a screenplay is a balancing act, and while all the components need to work together in harmony, the one component that can really make or break the script (in my opinion) is the character. In other words, you could have the most beautifully written story arc, and perfect structure to your script, but if your lead character is dull and flat then everything else falls to pieces. Conversely though, you may have story that needs work and the general pacing may be off, but if your lead character is compelling, your audience will remain engaged in the film regardless of some of the flaws it may have. In ideal world, you want to find that balance I mentioned above, where all of the components are working together harmoniously, and one sure way to start out in the right direction is to first focus on writing layered characters with meaning and purpose.