He had signed on after reading the script by brothers Duncan and Robert Sarkies, based on Duncan's 2008 novel, written while his older brother Robert made the acclaimed Out of the Blue about the Aramoana shooting. McKenzie had known Duncan for many years - he directed the first Wellington show that he and Jemaine Clement had appeared in together, and he had also written some Flight of the Conchords television episodes.
In Two Little Boys, McKenzie plays "Nige", an Invercargill bloke who gets into a spot of bother after running over and killing a Scandinavian backpacker.
Freaked out - "basically Nige is freaked out for the whole film. I must say 'f***' at least 100 times in that movie" - Nige turns to his best mate Deano, played by Hamish Blake of Australian comedy team Hamish and Andy. Deano is only too keen to help out, being the disturbingly loyal friend he is.
Madness, a botched body disposal and a trip to the Catlins ensues, which also drags in Nige's better-adjusted new friend Gav (Maaka Pohatu).
As a movie, it's not FoTC meets Hamish and Andy.
It's much more a film by the guys behind Scarfies, the 2000 local hit the Sarkies made about Otago student flatmates taking on the local drug underworld.
All of which meant for both McKenzie and Blake, it was time to act - and away from the safety of their respective comic foils.
"It felt a bit like we were cheating on our comedy partners," says McKenzie, "It felt like we were going off having an affair. But it was really interesting finding that rhythm with another comedian.
"Both of us come from a fairly solid background of being ourselves. So both of us were slightly in the deep end."
Robert Sarkies agrees that his casting of the pair may help audiences buy into the oddball two characters even if they don't like them much.
"I did feel for both of the characters, especially the Deano character - he's borderline psychopath really - that we needed an actor in that role that could let us in, we could feel for, who essentially felt like a nice guy."
McKenzie was intrigued by the balancing act the material required.
"The hardest thing I think, was trying to bring the reality of having killed someone but try to keep the comedy in the film. Because it's such a dark subject. Our aim was to make it feel real enough that you wanted to follow the story. And we didn't want it to feel like Weekend at Bernie's, which would be the worst case scenario - that it would become a sketch about getting rid of a dead body."
For Sarkies, Two Little Boys was a chance to cut loose after Out of the Blue and helming sci-fi television drama This Is Not My Life.
"It was sort of an audacious crazy story, the kind of films that they used to make in the 70s. Some kind of strange stoner buddy movie. I had just come off making this tragedy, so maybe I needed something lighter and odder."
The original idea for his brother's novel had been a joint one. And when he read the manuscript of the first draft he scrawled on it: "It's a film!"
But there was some adapting to do - the book was largely a first person narrative while the film uses a small amount of voiceover.
Sarkies also decided to shift the story back to the mullet-required years of 1993, to give the audience a different perspective.
"I didn't want people to judge the actions of these characters too directly. I think it's easier to look at the actions of your past. The actions of the characters are sometimes quite extreme but if it's set in the past it's got a little bit of a nostalgic glow to it.
"And add to that, you can have a lot of fun with the art direction and the wardrobing. Some scenes in the film are pretty full-on serious scenes. But the seriousness is undercut by a ridiculous costume that one of the characters is wearing."
The film also flashes back to previous decades and - for reasons best explained by the film - to the trenches of World War I, complete with Rolf Harris on the soundtrack singing the song from which the movie takes its title.
And it spends quite a bit of time in the lovely landscapes of the Catlins. Though despite shooting there in summer, those actors suffered in their jocks.
"The film looks a lot warmer than the shoot was," laughs Sarkies, "even though it was the middle of the summer. Quite often they were in their underwear, which we didn't do as a selling point of the film, by the way, it just so happened.
"When you put little thin Bret, medium Hamish and massive Maaka in a row in their underwear, its like some strange pub joke made for cinema."
But most of the laughs in Two Little Boys come with the rider: Should we be laughing at this?
Which is exactly what Sarkies was after.
"I knew I wanted to make a movie that played in the extremes and was extremely funny while also extremely dark, striking a tone that would be unusual but would take an audience along on this crazy rollercoaster ride and keep them with these characters despite their intensity and heinous actions.
"Striking the right balance of tone was the trickiest thing between comedy and tragedy. I didn't want one to overwhelm the other. I never set out just to make a broad comedy. I wanted to put people into the shoes of these characters which is an unusual place to be in.
"Sorry New Zealand, here's a perspective that you are not used to. Hope you enjoy it."
Who: Bret McKenzie and Robert Sarkies
What: Two Little Boys, a black comedy based on the novel by Duncan Sarkies also starring Hamish Blake of Aussie comedy duo Hamish and Andy.
When: Opens at cinemas on September 20]
Also: McKenzie and Blake are doing a post-screening Q&A at Event Cinemas Queen St on Wednesday, September 12 (6.30pm); the Sarkies brothers are doing one after a screening at the Rialto Newmarket on Thursday, September 13 (6.15pm).
-TimeOut
