He can do Pennywise's creepy eye trick without CGI
Pennywise is a physical role. But thankfully the power of special effects makes some of the more mystical elements of his character achievable without casting an actual magical sorcerer. (Also, when he shape-shifts in the movie, other actors helped play the titular menace.) But, according to interviews, there was one physical aspect that Skarsgård had under control: the clown's menacing stare. Apparently, Skarsgård is able to move his eyes in some pretty disturbing ways.
"I told Bill, 'I want it in both directions, I want you to send your eyes looking away and give a really unsettling look,'" said director Andy Muschietti's in a recent interview with DigitalSpy. "He said, 'I can do it', so he did it. What are the chances? It's one in a million and he was cast already. ... So even in those moments when his eyes move, it was in. In the storm drain when he goes limp with his eyes looking fucked up, that's all him."