Andres Muschietti was blown away by Bill Skarsgard's portrayal of Pennywise in the 'IT' remake.

The 44-year-old filmmaker has helmed the new adaptation of Stephen King's 1986 horror novel about a child-killing monster - who was originally portrayed by Tim Curry in a 1990 TV mini-series - and Muschietti wanted to make Pennywise ''unpredictable'', something his lead actor did to great success.

Speaking to Den of Geek, the filmmaker said: ''In part, it is the essence of the monster in the book, but it's not in your face.

''But I did want to bring out that layer to the newer version, so you see a character that is edgier because he's unpredictable and he does weird things.

''In fact, one of the descriptions of the character in the book is that IT was not very good at replicating human emotions.

''And that's something that is overlooked in general. In the 1990 adaptation you see Tim Curry and he is consistently rowdy and loud and pretty human, for a child murderer.

''And I wanted to bring that sort of glitch, erratic behaviour, with the purpose of making him unsettling.

''Then it was Bill, who with that concept started building and building and building. And it came to a point where he was bringing a new thing on each take, because unpredictable means unpredictable.

''He's an actor that is not afraid of bringing a new thing every time. So each take he would do something different and it blew my mind. It was quite terrifying.''

'IT' follows a group of youngsters who are faced with their biggest fears when they square off against the evil clown in Derry, Maine, after multiple children go missing.

The surviving kids mature into adulthood and are forced to revisit their horrors as one by one they encounter Pennywise again.

Muschietti previously revealed that 'Allegiant' star Skarsgard, 26, was so frightening in his full makeup that he freaked out his child co-stars on set.