Watch all six Star Wars movies in order and pay particular attention to Anakin Skywalker and Darth Vader's actions and emotions. Note his descent to the dark side.
Go over the script with the writer and director. Know what their intentions are for the character. The Darth Vader that is seen in the Star Wars movies may not be exactly the way a director or writer wants you to portray him.
Try to understand the feeling of losing the people you love. Anakin Skywalker's mother was killed by Sand People and then his love Padme dies giving birth to their children. Losing the most important people in his life allowed him to embrace these feelings of rage and hatred for everyone else.
Note Anakin's personality as a rebel. If you watch Star Wars: Episode I - The Phantom Menace and Episode II: Attack of the Clones, you will notice that the young Anakin has a problem with authority and following rules. He is arrogant and careless with his actions. Listen to Obi-Wan Kenobi explain that these are not the ways of the Jedi. "Characters, just as real people, are defined by their actions, by what they do and what they don't do," according to Linda Cowgill for Plots Inc. Productions.
Empathize with Anakin's struggles between good and evil. Anakin believes that the only way to save Padme's life is to turn to the dark side. Anakin is told by Darth Sidious, "We must move quickly. The Jedi are relentless; if they are not all destroyed, it will be civil war without end. First, I want you to go to the Jedi Temple. We will catch them off balance. Do what must be done, Lord Vader. Do not hesitate. Show no mercy. Only then will you be strong enough with the dark side to save Padme." In Star Wars: Episode VI - Return of the Jedi, Darth Vader has a change of heart and sacrifices himself to save his son, Luke.
Acknowledge Darth Vader's physical pain as well as emotional hardships. At the end of Episode III: Revenge of the Sith, Vader is badly burned and left to die by Obi-Wan Kenobi. He becomes so disfigured he has to wear a full-body suit, complete with a breathing apparatus, which is what we see him wearing in the last three episodes. The body remembers pain and reacts to it, even if remembering a moment of pain or weakness, according to Neva J. Howell of Ask a Healer. In order to become Darth Vader, you must associate his pain with pain you have felt.