The folks over at Computer and Video Games have an interview with Kevin Stephens, Monolith's director of technology.

How did the supernatural element creep in?
Kevin Stephens: When we were working on the core concept of the game, we felt it was great but generic. It was just action, and even though it was stylish high concept action we thought it would be nice to add some more flavour to the action. Then we started thinking 'What can we add?' and a lot of people on the team were into horror and stuff, they were into The Ring, Ju-on, Darkwater and all these great Japanese horror movies.

I don't know when the moment happened but someone brought up the idea, we thought it would be good so we talked about how it could work and, as we didn't have a story at that time, it just evolved. And we fell in love it and it started taking over the action element. So now they fight it out - the action versus the horror.

I for one hope this game lives up to it's expectation. Though people are going to be sceptical after all the hype Doom3 had just to leave people feeling disapointed. This game certainly sounds like it could be very satisfying.