The new movie adaptation of Stephen King's 1986 novel It has broken box-office records in the United States in its opening week, but what makes it so special?

And It is special - the horror film about a shape-shifting demon that hunts children has grossed more than US$120 million (S$161 million) in the US in its opening week, breaking records for an R-rated film and a horror work.

On the surface at least, the success of the film - both commercially and with critics - can be attributed to how well the movie marketing machine works. The studio behind it, Warner Bros, would have screened it to test audiences and edited bits that did not work.

It also bought the rights because it knew about the book's iconic status and saw a pent-up demand for a film update.

Then, after the trailers were released earlier this year to record viewings, Warner knew it had a winner on its hands and made sure the film got as wide a distribution as possible across the US on opening night. Ker-ching!

That is the simple answer, but it does not explain other things.

Why did this mid-budget movie with no stars work so well, when other King adaptations, such as the recent The Dark Tower (2017), starring Idris Elba, did not? Why did another mega-budget horror work, The Mummy (2017), featuring Tom Cruise, do so poorly in comparison?

I think it has to do with what horror and monster movies have that other kinds of movies don't: the reveal.

It director Andy Muschietti very smartly figured that Pennywise the clown was the star of the show, just as the shark was the star of Jaws (1975) and the xenomorph the star of the Alien franchise (1979-2017).

Do the reveal of the star right and you will have people coming out of cinemas clutching their chests and dying to talk about it, which is what happened with It.

Also, the movie played with contrasts very well. The kids' ensemble at the centre, the Losers' Club, are a rib-tickling lot - there is the overweight one, the asthmatic one, the foul-mouthed one and so on. The moments the audience shares with them at play, when juxtaposed against their desperate fight in the sewers, are starkly different.

In The Dark Tower, which ended its run here recently, the tone is, to use the fan slang, grim-dark. Yes, it had a kid character in it, but the tone was unrelentingly dour, when it was not trying to be cool. That Christian Bale-Batman vibe is very hard to pull off and even director Christopher Nolan knew it - his Batman films have much more humour and heart than most realise.

The next factor in It's favour is special effects. Horror movies are the new action franchises, it seems. In the Taiwan ghost possession movie The Tag-Along 2 (still showing in cinemas), the last act is a computer-animated gongfu battle. It is as scary as a sleepy poodle.

Horror movies run on fear, and fear comes from the meaning that is attached to things. In It, a red balloon and a modernist portrait of a woman are not scary in themselves, but they become scary once the audience works out what they mean.

In Japanese horror film Ring (1998), a video tape and a television are frightening (the Sadako creature that emerges from the TV is scary, but she is seen only at the end, and only briefly).

When computer creatures appear, they are scary for about five minutes. Once you get past the big eyes and teeth, they become just another monster to be killed.

Finally, the idea that horror is the new comic-book action hero movie is taken to its natural conclusion in the Dark Universe franchise, which saw the release of The Mummy earlier this year as its launch movie.

The big-budget The Mummy earned US$400 million globally, and is an underperformer when compared with Cruise's Mission: Impossible - Rogue Nation (2015), which picked up close to US$700 million worldwide.

Bloated with stunts and special effects, The Mummy wants to be a kid-friendly horror movie for people who do not like to be scared.

The It adaptation goes the opposite way. In Singapore, it is rated NC16 (no one under age 16 is allowed) and has a scary monster that kills kids.

It opened at No. 1 here last week.