A lot of beginning screenwriters ask me how to establish their main character's arc. Here's the easy answer: Know your ending! Why so important? Because it's a tremendous help to know the final destination. Imagine that you're traveling to your in-laws' home in Ohio for Thanksgiving dinner, but you have no idea in what city they live. You're driving blind; it's impossible to plan your route.
But if you know your ending, you can use that end as a guide to dictate how you begin. And this matters, not only for plot and screenplay structure, but also in developing character arcs. For example, if you want your hero to obtain his or her objective - to be victorious at the end of the script – it makes sense to start him at a very low place in the beginning of the film as well as illustrate his major character flaw(s). This way, the journey will be huge – for the character and for the audience – and the character will learn to overcome his flaw(s) along the journey, becoming a better (or worse) person because of the experience.
A great example of maximizing the journey and arc is illustrated quite well in the 1976 Academy Award winning Best Picture Rocky. In the opening scene, we see Rocky, a washed up "bum" fighter, struggling to finish a fight with Spider Rico in the decrepit Blue Door Fight Club, a place screenwriter Sylvester Stallone wrote "resembles a large unemptied trash-can." During the fight, Spider head butts Rocky, opening a bleeding cut on the corner of Rocky's eye, a foul that drives Rocky into a flurry of punches, landing a knock out to Spider's jaw. After the fight, in a dingy locker room that a cockroach would have issue with, Rocky recovers all alone as the promoter pays out his measly winnings.
Cut to... almost two hours later. Rocky and Apollo Creed have just completed a 15 round bout for the Heavyweight championship of the world in front of a crowd of thousands (millions watching on television), Rocky receiving a split decision with the King Kong of fighting heavyweights. And during this shining achievement, all Rocky can call out for is "Adrian", who makes her way into the ring where the two embrace.
Talk about a journey. In the beginning, Rocky was a washed up fighter, broke, and alone. But in the end, he goes toe to toe with the Champion of the World, his financial problems behind him, but most importantly, he has Adrian's love and a life together to share.
Knowing the end really is the key to both determining how to begin your screenplay as well as helping to plot your hero's journey and developing his/her arc.