Pennywise the dancing clown is such an iconic monster in fiction that you likely know a great deal about him, even if you've never read the book or seen the movies based on IT. Bill Skarsgård plays the role in the new film, and was uniquely suited to play the role. The director of the new IT adaptation, Andy Muschietti, says that Skarsgård had two specific traits that he brought to the role of Pennywise which he knew the character needed to have. According to Muschietti...
Unpredictability is key to any sort of horror material because the way you put a scare in your audience is to give them something they don't expect. In the case of IT, this was done by giving Bill Skarsgård free reign to experiment with Pennywise and try different things in order to find what worked, as Andy Muschietti explained to MTV. This certainly would have added to the character's unpredictability, if even the director didn't know what the actor was going to do, then the audience would be in the dark too.
The madness of Pennywise is also clear on the screen. The previous version of Pennywise adapted for the screen was a more cold and calculated evil. That Pennywise was clearly in control of itself. This one seems much more insane. You never know what the creature is going to do, only that you won't be able to guess what it is. The fact that Pennywise is a supernatural creature doesn't even need to enter into the equation, as everything that makes Pennywise terrifying in the new film can be linked to that erratic madness.
Pennywise had the opportunity to scare a lot of people this past weekend, as IT blew away all box office expectations and made as much in the first weekend as it was originally tracking to make in over its entire run. The film has also seen significant critical success, making IT look to be one of the great adaptations of Stephen King ever brought to the screen.
While IT is clearly classified as a horror movie, it's also a monster movie, and while IT isn't really about Pennywise anymore than a Godzilla movie is really about Godzilla, if the monster doesn't work, the movie falls apart. Pennywise works as well as any big screen movie monster, which allows the movie to focus on everything else, which is where IT is able to become a truly great film.
Bill Skarsgård will have a whole other opportunity to scare the hell out of us now that the second half of the story of IT will almost certainly be made.