The Swedish star, 27, takes centre stage in the new adaptation of Stephen King’s iconic horror.
Speaking to People, he recalled what it was like to suddenly finish filming and return to normal life.
“It was weird,” he said. “I really enjoyed myself doing it, but the day after we wrapped, I went back home to Stockholm, Sweden, and it’s always a weird experience when you work really hard on something and then you go back to regular life at your childhood home.
“And that’s when I finally realised what an impact the character had had on me.”
He elaborated: “For the following two weeks I would have these really disturbing dreams about Pennywise.
“It was very strange, it was almost like a really slow exorcism to completely let go of him.
“The closest thing I can liken it to is being in a very destructive relationship, because you never really realise you’re in one until you’re out.
“And then as soon as you’re out of it, you’re like, ‘Oh my God, I can see it all so clearly now.’”
It is set to enjoy huge box office success upon its release today, and among its fans is King himself.
“I had hopes, but I was not prepared for how good it really was,” the writer admitted.
“It’s something that’s different, and at the same time, it’s something that audiences are gonna relate to.”
It is out now.