The horror star, 27, told Conan O’Brien on his US chat show that it originates from him terrorising his younger brothers as a kid.
“It’s a funny story because my older brother, Gustaf [Skarsgård]… he had this ability to point his lip in a very strange way,” he recalled.
“For some reason — I don’t know how — I understood that I could do it as well.”
“But I was like maybe 10 years old and I’m like…,” he added, pulling the unsettling face.
He went on: “I would pull my T-shirt up over my head so you wouldn’t see my… just the [eyes, nose and mouth]. And I would make this face and chase my little brother and scare him.
“He’s traumatised seeing the film. ‘Not again!’”
It has had a huge reception at the global box office, romping to over $180 million worldwide.
It’s also expected to stay at No1 for a second week, beating the likes of Mother! and Victoria and Abdul.
It’s not just Skarsgard who is spine-chilling in It: yesterday the actor who plays little six-year-old Georgie was seen in a disturbing video for a US cinema.
He recreated the famous “You’ll float too!” line from the movie and its promotional materials.
It is out now.