The movie kicks off where many rom-coms might feel they've reached the end of their story - at the wedding - and it quickly becomes obvious that Josh and Nat have their differences. Josh performs a surprise novelty rap dance routine at their wedding while Nat would rather have had an intimate, conventional first dance - but they've succumbed to an undeniable spark and think they're deliriously in love.
Nine months later, it's evident those differences aren't quite as trivial as they thought, and are slowly driving them apart and into the office of a somewhat deranged marriage counsellor (played by an excellent Olivia Colman).
Neither Josh nor Nat want to throw in the towel, but they're also not sure how to make it work - especially when they both realise they're attracted to other people.
While Byrne has most recently been seen alongside Glenn Close in legal drama Damages, she's also made a name for herself in the comedy world with roles like the very naughty, sly-winking Jackie Q in Get Him To The Greek, and the competitive bridesmaid Helen in Bridesmaids. But Nat's straight-laced nature and exasperation at Rafe's mile-a-minute quips was one of the more difficult characteristics she had to adopt.
"Nat is very different to me, so that was a bit tricky, it was a bit counter-intuitive. Nat's quite humourless, weirdly enough, and that was strange trying to tap into that quality. I feel like I'm usually quite a good audience and I love laughing, I laugh easily, and Nat's not like that at all, she's much more controlled and tense."
Which is especially challenging when you're among such a comedic cast - including Minnie Driver as Nat's sharp-tongued sister and Stephen Merchant as Josh's highly inappropriate best mate.
"Stephen Merchant is hilarious, and he improvs a lot, so some of that stuff would come out of the blue. And Rafe Spall is hysterical, too, he would make me laugh all the time, constantly in stitches."
Improvisation is something that Byrne has become increasingly familiar with, having worked with comedians like Russell Brand in Get Him To The Greek and Kirsten Wiig in Bridesmaids - two films in which improvising was encouraged, and both she and Spall were given room to play when working with Mazer, too.
"This was more like Dan pitching me different jokes as we did each scene, different ideas, that was more the style of improvisation that we had happening, but I did do a bit. Rafe probably did more improv than me but we were certainly allowed to try things. I have grown into improv over the last couple of years, I'm more used to it now and I've learned to trust my instincts a bit more, but it's definitely been a learning curve. I have loved doing it though, I've wanted to do comedy for a while so these opportunities have been really exciting."
Byrne seems to have a taste for broad, somewhat risque comedies which push the envelope a little - whether they're making a music video as a potty-mouthed pop star or accidentally showing their parents raunchy honeymoon photos, her characters always have a hint of sass about them, which Byrne enjoys.
One scene she didn't find so enjoyable this time around involved a pair of flying doves let loose in a hotel meeting room.
"That was really hard, mainly because I was so scared of the birds. But it actually ended up being very funny, so thank God it worked. It was a tough day, though ... My discomfort was certainly in the script but perhaps not to the extent that I was experiencing, so it took a while for me to get a handle on it - but in some ways maybe it made the scene better."
Byrne was clearly committed to the role then - she and Spall even spent two hours with a real-life marriage counsellor, pretending to be a real couple in order to make themselves as relatable as possible in the hilarious heightened reality of the film.
It's a tricky line to walk, trying to keep the audience on side while also highlighting some of your most annoying traits, but Spall and Byrne both do a first-class job, arguing over things like emptying the rubbish bin and misheard song lyrics, all the while keeping us sympathetic to their situation.
"I think as an actor you've got to be your character's lawyer, in a funny way - you're presenting their best argument in the case that you're fighting. So you're on their side, you're empathetic to their situation, to them; everyone deserves a fair go. And I know that on the page Nat might not seem that empathetic, but I find it far more interesting to play a character like her than someone who's a people-pleaser."
Who: Australian actress Rose Byrne
What: British rom-com from Borat and Bruno director Dan Mazer, I Give It A Year
When: In cinemas February 28
- TimeOut
