STU RUTHERFORD: He's handy at writing computer code and advising on software requirements, and also, as it happens, has a knack for acting.

It's out of the shadows and into the spotlight for IT guy turned actor Stu Rutherford.

At film festival screenings of their new movie, the fans don't reserve their loudest cheer for celebrated writers and co-stars Jemaine Clement and Taika Waititi. Instead, says Clement, they "erupt" when "Stu the IT guy!" wanders on stage.

Other than a 30-second cameo involving a fork being jammed in his face, Clement and Waititi's smart vampire mockumentary What We do in the Shadows is 38-year-old Stuart Rutherford's first screen credit. He is, however, quite handy at writing computer code and advising your business on its software requirements.

SHADOWY: The cast had no idea how the movie progressed until minutes before their scenes were to be staged.

What apparently began as an elaborate gag - casting an IT guy named Stu as an IT guy named Stu - has created one rather unlikely film star.

Rutherford is a part-time business analyst for a Wellington company, LanWorx.

"They are a bit weirded out that this guy seems to pop in and pop out a lot," he says of his colleagues. "They've been asking ‘How big is your role?' I told them it was quite small. One of them said ‘wait a minute, you're on the poster'. I said there was quite a lot of people on the poster."

Actually, Rutherford sits fifth on the credit roll - behind only Waititi, Clement and fellow vampires Jonny Brugh and Corey Gonzales-Macuer. And as the only human character in the central cast, Stu becomes rather the hero of the hour: something he didn't realise until he began shooting the penultimate scene.

"When we wrote the script and made him a big part of it," relays Clement, "we let him think he was going to be our IT guy, and told him he'd just be in a little bit. Every day he'd go, ‘So when do I help with the computers?'; and we'd say, ‘Oh, just put that costume on first.' Because we almost keep him silent, I think, the whole time he thinks he's just being made fun of."

Rutherford, who had chanced into a bit part in the original 2005 short film on which Shadows was based, explains with the trademark precision of his other profession: "I assumed I would be in it slightly more - if you take a 20-minute short and expand it out to one hour 30 minutes, and you've been in for 10 seconds, you expect to be in for maybe 40 seconds."

On the phone from New York, where's he's on a long-booked holiday, having not known just when the movie would be released, he adds: "I imagined my role would be quite small - standing in a corner, drinking a beer. Because acting is the total complete opposite of IT."

Rutherford didn't, he admits, show much interest in acting until about 2004, when he went flatting with Waititi, an old Onslow College schoolmate, and a bunch of actor-writer types.

He qualifies this. Well, he says carefully, do you remember the Garbage Pail Kids, who were the evil version of the Cabbage Patch Dolls? When he was in primary school, Rutherford and his mates would perform little plays based on them, as Smelly Simon and Gross Graham and Slimy Stu. He did some more stuff at intermediate school. But at 13, he "got all serious" and ended up studying information systems and Japanese language at university.

So, anyway, he was in the flat in Mt Victoria and Waititi, starting work on the original Shadows, asked if he could store a coffin in Rutherford's bedroom for a few days. "Then they would say ‘we're heading into town, want to come' and I ended up in a couple of scenes," he says. Talk began that a full-length version would follow and that was enough to spur Rutherford into action, auditioning - mostly unsuccessfully - for advertisements to get some screen experience.

Then he thought maybe some time behind the camera would help, so when Waititi gave him a job as a runner on his 2007 charmer Eagle vs Shark, Rutherford quit his day job writing computer code. He felt he needed to lift his game, so considered the essence of being a "runner" (effectively a production crew skivvy, the most junior staff member) and bought two retro Adidas tracksuits in red and black: "I wore the black one on my first day, then turned up in regular clothes the next, and they told me to wear the tracksuits every day."

It was he, says, "quite fun, a great way to learn what every department does and not have too much responsibility - you have a list every day, you clear it, you go home. If you're a producer, you're constantly worrying about weather, and animals and funding."

Next came Waititi's big hit, Boy (2010) in which Rutherford was upgraded to production assistant and, benefiting from the remote filming location at Waihau Bay, secured a cameo role. He earned his Internet Movie Database (IMDb) entry for playing a prison guard stabbed with a fork by Waititi during a dream sequence where James Rolleston's title character is imagining his wastrel father escaping jail.

He was in another short for a friend. He has spotted two patterns: "Generally, I am in a uniform, or there is some kind of blood", and "all the jobs I've got have come through people going, ‘What about Stu doing it?"'

Except one.

Proudly, he says his biggest claim to fame is appearing in a Peter Jackson short film.

Unfortunately, the only place the World War I doco Over the Front (2008) screens is at the Australian National War museum in Canberra on a special 21m x 3m screen.

Rutherford was upgraded from extra to Sopwith Camel fighter pilot, instructed to sit in a cockpit before a blue CGI screen as Jackson bellowed "dive, dive, shoot, shoot". Rutherford thought: "Wow!"

As Jackson demonstrated how to climb into the cockpit, his shoe fell off and Rutherford picked it up. "So I can," he declares, "claim to have touched Peter Jackson's shoe."

Even as he learned more about the industry, Waititi didn't want his old mate to change.

"At one point," he says, "I got an email from Taika all in capitals - ‘DO NOT DO ANY ACTING COURSES.' "

He realised that Waititi wanted to preserve the "2005 version" of Stu intact; Rutherford implies that his career shift since then has rather changed him.

"One time Jemaine said, ‘Would you really wear those shorts?' I was like ‘Erm, you're right, I wouldn't.' I felt bad for costume [department] because I told them I would. So I thought ‘OK, Stu, make up your mind. So the touchstone for me was ‘Would I really do this?' "

He says it has given him a new appreciation for how hard acting is - if he found it taxing playing himself, what's it like playing someone else entirely?

Clement and Waititi's decision not to share the script with the rest of the cast may not have helped. The result was 120 hours of footage and instructions to Rutherford along the lines of "sit in the kitchen, and react to what we say to you".

Crew, he says, were told not to tell him anything - "it was like we had a funny disease" and so not until 10 minutes before the movie's pivotal scene did he know what was to pass. And it wasn't until they all went to the Sundance Film Festival and saw the finished version that he realised the magnitude of his role.

Despite the raucous response at screenings, Rutherford seems entirely unruffled by the prospect of fame - "all I need to do is grow a beard, and I'll be OK" - or Hollywood attention.

"I'm possibly more excited about where people like Johnny and Corey and Jackie [Van Beek, who plays Brugh's human familiar] will go: it's going to be awesome for them." He says he spent plenty of time at Sundance redirecting people to YouTube clips of his castmates.

So now he's working part-time, going to a lot of auditions and, with a friend, devising a film-crew lighting system he can't talk more about until they get it patented. It's possible, he finally concedes, that Shadows may produce more regular work.

"I am open to more acting," he says. "At the same time, I realise I don't know what chance there is of any more movies being written with a role based around me. It's probably zero, to zero point five."

What We do in the Shadows is released June 19.