They gave the world Saw, followed it up with Dead Silence and now writer director team Leigh Whannell and James Wan are back with the terrifying ghost story that is Insidious.

So what is Insidious about?

Leigh Whannell: The film is our version of the classic haunted house movie - we are huge fans of the classic haunted house genre but we wanted to bring something new to it and make our own version, so we came up with an idea that we thought we hadn't seen before and it's essentially the story of a young family who moves into a new house, things start to move around on their own and go bump in the night as they do except that the twist occurs when the eldest son falls into a coma and doctors can't figure out what's wrong with him and you figure out there's a connection between the scary stuff and the boy.

What first drew you to horror? What was it that appealed to you about the genre?

James Wan: Leigh and I, after Saw, we pretty much became known as the Saw guys and I took some time off after my last film - Death Sentence, the Kevin Bacon film - after that I was pretty tired, pretty exhausted, pretty stressed out so I took some time off and after that I said to Leigh: 'Dude I would love to come back to a movie but I don’t want to come back to a movie where I have no control, I want to come back to something that I know I can get the funding for, that people can trust us’, and within the horror genre is where we've really established our names. However I did not want to do something like Saw again, I wanted to do something different and since we're such big fans of the haunted house genre why don't we try that, try and write a really scary haunted house film and let the movie just speak for itself. Haunted house movies have been done to death in some ways but I believe we can bring our own take and vision to it and that was really the genesis and it was great - we were able to marry this really cool concept superimposed onto this haunted house sub-genre.

There are a lot of haunted-house films out there and most of them actually aren't scary but Insidious is. What do you think it is that magic ingredient?

LW: We know what scares us. A lot of the scenes in the film are scare-scenes based on stories that we've heard from our friends and families.

JW: When we first heard these stories we would have chills going down our spine and so we thought ‘well if that's how effective it is just hearing the story, imagine putting it on screen where you can control the framing of the shot, add music to it, dim the lights, have the tools of cinema at your hand to really control and design the whole scenario.’

LW: It's like writing a comedy, if the first time that you're writing a joke it makes you laugh then there's a good chance when it finally gets to the screen the audiences will laugh at it – hopefully. And that's kinda how we treat horror, we look at what scares us. Of course, by the time we get to the editing stage and the shooting stage it's no longer scary anymore because we've seen it so many times but we can cast our minds back to the first time we heard that story and, as James said, you just get these chills, these goosebumps.

JW: And the other thing I want to add to that is, I think why Insidious is very effective as a scary movie, is because as you're watching the movie never once did we throw in any fake scares. I think that is very important, not having fake scares meant that whenever we build to something, something's around the corner, something bad's about to happen it is not a good thing - it's not the wife is about to bump into the husband, a cat jumping out or something like that.

LW: The audience isn’t trained to expect being let off the hook.

JW: Exactly. And so having no fake scares meant that anxiety just builds and builds, and having the score, the particular score that we designed for this film is super effective as well with setting people on edge. I wanted the audience to watch the film with the musical score underneath it to basically get an anxiety attack, to feel really anxious as they're watching the film because I think ultimately that's what makes people cling on to their seats or onto their friend next to them. I think that's what gives it its power.

Did you treat the score almost like another character?

JW: Yes. We worked very closely with the composer [Joseph Bishara]. We really wanted a very atonal, almost avant-garde sound design score to the film. Having a non-melodic score meant that the music itself doesn't give you comfort. The film doesn’t give you comfort, the music gives you even less comfort actually. The music if anything is so jarring. You go from really quiet moments to screeching violins and it's so jarring and I think that really constantly sets you on edge and adds to the anxiety factor.

How did you first meet, and how did you realise that you would work together rather than just be mates?

LW: Well we met at film school so we both had the same goal - we wanted to be involved in film-making.

JW: Leigh realised he needs me [laughs].

LW: James realised his writing skills left a lot to be desired [laughs]

LW: Essentially we were mates first, we kind of bonded over this shared love of Evil Dead and Tim Burton and Mario Bava and all that crazy of stuff and then we finished film school, we would go to film festivals like Melbourne International Film Festival and just devour these films, from Spain, from Japan, from everywhere - we loved it so much and then we just decided to make a film together and it was great because we're not stepping on each other's toes you know, we have respect for each other's craft. I'm writing the script and James is directing and we're both fans of each other's work. James loves the stuff I write and I love what he directs so it's a good partnership in that way because the boundaries and skill-sets are defined, as opposed to if we were a directing duo and so I think that's what made us a natural fit to hang out together. I'd rather be writing a script knowing who's going to direct it rather than doing it on my own. I'm sure James would prefer it to have someone write a script for him for free …

Do you find there is less pressure when you're working together?

JW: I find it very comforting - the great thing is just even having Leigh on set, that's why I like to have Leigh play characters in my films. I just like to bounce ideas off of him even when we're filming and it's great especially if he wrote it. I love Leigh being in all the movies that I direct even if he didn't write them.

LW: It's good, I think there's power in numbers. I think Hollywood can be a really massive kind of snake-pit, you have to watch your back at all times.

Going back to Saw did you ever expect such an extreme film to become mainstream?

JW: Here's the crazy thing, after we played the movie at Sundance, I always thought that I had directed a movie, made a movie that would only play well to a niche audience, to a cult, horror fans. But what we discovered was, we got some pretty snarky reactions from the hardcore horror fan community but it was the mainstream public that really embraced the film. They were like ‘Oh my goodness we've never seen a film like this before with such a crazy twist, with such a crazy villain with traps and all that stuff.’ They were the ones that embraced it and that's how it ended up becoming such a huge hit, because we broke out of that niche audience and we hit the mainstream audience which is what every studio really wants when they make a film is to make a movie that could have mass appeal to the mainstream public and we just never thought that we could do it with such an extreme horror film.

LW: The extreme became mainstream.

I think it changed cinema, a film like Antichrist would never have existed without Saw

JW: Interesting for you say that, to quote a very art-house director [Lars von Trier] as being influenced by the low brow.

LW: I think Antichrist is totally coming in the wake of Saw.

JW: Maybe they realised that for it to be controversial they needed to go beyond what the public has seen before.

LW: Now you're seeing films like Human Centipede, A Serbian Film - to push the envelope they need to go places that are just out of control, that get people put in prison [laughs].

Finally will we see a sequel to Insidious?

LW: No. It's not that we're saying 'no sequels', it's just that we haven't put any thought into it because we're still so focussed on the release of this one and we're way too superstitious to talk about a sequel.

Insidious is on general release from Fri 29 Apr.

Insidious Trailer