Toni Collette is getting riled.

The Australian actress doesn't like it when you refer to her characters as mothers. Even if some of the best roles she's played - think Little Miss Sunshine, The Way Way Back or her Oscar-nominated turn in The Sixth Sense - have all been matriarchs. "Mothers are still women," she says.

"You know what s***s me? No one says to Jon Hamm for Mad Men: 'Oh, you're playing a father.' No one focuses on that with guys. It really p****s me off. Sure, I play mothers, but these people have things going on in their lives. They're whole, complex people."

It's a fair point, made with typical bullishness by the 41-year-old Collette, who arrives for our interview dressed in black trousers, a beige jacket and a T-shirt designed by her "friend", photographer Thomas Dozol. We're in Berlin, where her latest film, A Long Way Down has just premiered at the film festival.

It's the second time she's been involved in an adaptation of a Nick Hornby novel, following 2002's About A Boy.

"The types of movies I like to do share the tone that he writes in," she says. "There's a real poignancy and depth but there's also great comedic moments."

The comparisons don't stop there. In About A Boy, Collette played Fiona, the hippie-ish mother who tries to take her own life. This time, she's Maureen, mother to a severely disabled lad, who begins the film by contemplating throwing herself from a London rooftop on New Year's Eve.

"I know they're both suicidal women! But at least in this movie I got to be a bit funny," she protests. "There was nothing funny about Fiona, even though About A Boy was ultimately a comedy. Everyone else was a bit funny and I was a bit tragic. But Maureen is funny."

This time, the humour comes as Maureen forms an unlikely bond with three other potential suicides (Pierce Brosnan, Aaron Paul and Imogen Poots) she meets, somewhat coincidentally, on the rooftop that fateful night. Even more than her role as Cameron Diaz's long-suffering sister in 2006 film In Her Shoes, it's the frumpiest character Collette's played since she gained 40lbs for her breakthrough as the Abba-loving wannabe bride in 1994's Muriel's Wedding.

Here, she seems to have been quite affected by playing someone who wants to end her life. "I've never been in that position, where I think 'This is it, I'm outta here.' But I do think about death quite a lot," she says, softly. "It's a part of life, and I think it's inevitable, and I don't like it at all! I think the Buddhists have it right. If you don't ignore it, and maybe live with it a bit in your mind, then maybe it won't be so scary when it comes."

Collette is not one for self-censorship, as evidenced when we start talking about music. She's just played a depressed rock journalist in forthcoming indie Lucky Them ("I'm not a mother in that!" she chides) and the conversation turns to the best gigs she's ever been to: Arcade Fire, Radiohead, Patti Smith and one particularly eventful Wilco show.

"I didn't know I was pregnant with my daughter Sage and I was jumping up and down and wet myself, and I thought 'Oh my God!' And I was... I was pregnant." She looks slightly aghast. "I shouldn't tell people that!"

Married for 11 years to musician Dave Galafassi, Collette has two children now: Sage, six, and son Arlo, who turns three in April. Her kids weren't their only collaboration either: she and Galafassi also recorded the album Beautiful Awkward Pictures, with Collette on vocals and her husband on drums, under the name Toni Collette & The Finish.

There's been nothing since, mainly due to the pressures of parenthood.

"I do a little scribble here and there and sing into my iPhone once in a while, but you've got to have the time to do it."

The eldest of three, Collette was always destined for some sort of life performing. Her father reckons she "came out of the womb and there was a spotlight at the end of the tunnel", she says, grinning.

Born in Sydney suburb Blacktown, the daughter of customer service rep Judy and truck driver Bob, Collette took acting classes at school, then took to the stage in a Sydney production of Godspell - all in her teens. Then she told her poor parents that she wanted to leave school.

Naturally, they refused, but her compulsion to act was so strong, the headstrong Collette, then 16, forced their hand. "That's it," she says. "I was gone. And I loved school. I was good at school. I was just like 'I've found it'. It was like finding God. And on a good day, it still is. And also at that age, my balls were huge. I was so fearless. I had absolutely no doubt. I didn't doubt myself. I didn't doubt anything. It was just a pure compulsion. It was like a magnet." While it's been over two decades since she made her first screen appearance, in Aussie soap A Country Practice, Collette still hasn't lost any of her fearlessness. Up next are small roles in Hollywood comedy Tammy, alongside Bridesmaids star Melissa McCarthy, and Hector and the Search for Happiness, with Simon Pegg.

But then comes Glassland. "I think it's the most emotionally raw character I've ever played," she says. An alcoholic living in Dublin, she's also a mother. Just don't mention that bit to Collette.

A Long Way Down opens on March 21