Bill Skarsgard explained to Conan O'Brien how he developed the terrifying smile he used for his role as Pennywise in "It."

While appearing on "Conan," Skarsgard said he discovered the ability to make the creepy smile runs in the family after his brother and fellow actor Gustaf Skarsgard was able to do it.

"He had this ability to point his lip in a very strange way," he said.

At the age of 10, Skarsgard said he discovered he could contort his mouth in the same way and used it to torment his younger brother.

"I came up with this character called Jurgen and I would pull my t-shirt up over my head and do this face and I would chase my little brother," he said.

Skarsgard said his younger brother was OK after the experience, but joked that he was still traumatized watching him perform in It.