Spoilers for season 3
He suggested to the Duffers to include Billy's mom: "That was something I was insistent of having included, to add to his backstory and to see the pain his mother caused him by leaving."
On shooting the sauna scene: "It took a week to shoot that scene. I think that kind of scene is every actor’s dream. It’s up and down, up and down, with the emotional complexities. I lost my voice the most after all of those days — taunting the kids, screaming at them, crying at them, transforming into evil in front of them. To play a polar opposite version of a character from the previous day to the next day is very fulfilling. Thankfully, the sauna itself wasn’t hot, but it was filled with some steam and I had a lot of lip gloss on my face to make me look sweaty."
On Billy at the end: "I really hope people will associate Billy with what he did in the end. I really tried to humanize him as much as I could, especially with what I was able to get included about his parents. I tried to empathize with him as much as possible. It’s hard, but he’s a human being. We all have elements of good and bad in us, and to successfully portray such a conflicting personality was a rewarding acting challenge that doesn’t come around too often."