What was it like to embody this complex character in what he’s going through physically?

For “Flayed Billy,” I did a lot of research on bipolar disorder and split personalities, and how one personality controls the other personalities. This season, Billy is like a rubber band that keeps getting tauter, and I tried to convey that in my physicality. But if you look at my eyes, that’s non-Flayed Billy trying to come through. The whole season I felt like my eyes were bleeding because I was trying to push out this emotion to play as a counter to my physicality. I wanted to treat it more like a real-world experience. I think that’s why the show is so successful — it’s tethered to a science-fiction element, but it’s anchored very strongly in humanity.

As an actor, how do you convey an emotion that runs counter to your physicality?

I don’t want to get too meta, but I think people put on a mask when they leave the house. There’s a “Public Brian” and a “Private Brian.” We all put on masks, even if just to mask our insecurities. When you’re listening to someone and trying to mask feelings like lack of confidence, or if you’re being antagonized or bullied, trying to be strong — in my eyes, it’s still Dacre. He’s so taut. I needed to think of him as a rubber band pulled from the top and bottom.

How difficult was it to film the climax?

We filmed that over three days, and both Millie and I lost our voices every day because right up until the camera would roll, we would run through the mall and land on our mark and scream at each other until we both cried and cried. She has such an emotional maturity. I like to give 150 percent every time I’m on set, and she’s the same way. We just scream at each other and cry and talk about things. It was warm and collaborative, and I was lucky that I had an actor who could provide all that emotion. When she’s talking about my mother, it’s painful, and we had to keep having our eyes wiped down, for continuity, because of crying.

How much detail did you and the Duffer brothers discuss regarding Billy’s past in order to make that final moment with El so powerful?