Warning this article contains season three spoilers of "Stranger Things", do not continue unless you want to be hit with spoilers.
Billy is unlucky as he is taken in by the Mind Flayer here in Hawkins Indiana after the season two finale of Eleven closing the gate that leads to the upside down. Since Russians have created a top-secret activity base to try and reopen the gate, it keeps the mind flayer alive. Even by the end of season, we are still left with unknown questions such as why the Russians when trying to open the gate.
When the mind flayer takes over Billy, it is almost as if he is his own person but is trapped by a darker version of himself. He can't control what he does and acts. In order to portray the vast difference in personalities, Montgomery admitted in an interview that he researched topics such as Split Personalities and Bipolar Disorder. While we all know that these disorders that Dacre research is not displayed within the show, it allowed him to understand how to become two characters and play them well.
As the Duffer Brothers began writing up season three, they called up Montgomery and asked him what he wanted to see. From there he asked them to show more of a backstory for Billy because a villain couldn't become a villain without an explanation as to why. He believes that the audience needed to understand the pain Billy went through when his mom left and by exploring his back story involving the mother, it creates a glimpse of innocence and shows heartbreaking moments.
During the emotional season finale, Montgomery noted that working his character against that of Millie Bobbie Brown who plays eleven was more mentally challenging than anything else. They would even practice screaming at each other and acting while warming up to stay in character. After they would shoot scenes with such raw emotion they would even comfort each other as they wipe tears from their eyes.
Not only is his acting amazing, but Montgomery fulfills his own stunts including being dragged in the Steel Mill. According to an interview he stated that it is thrilling to do the stunts himself since many people do not and hire a stuntman. Unfortunately, Billy's hair in season three is all a wig. He personally enjoyed having the wig because of his character being beaten up and dragged around. I think almost everyone can agree that he should grow his hair out like Billy's!
We love Montgomery and although we are sad to see him go we all can't wait to see what he does next!