AS Cannes Film Festival competition slots go, being unveiled on the first Thursday is a double-edged sword.

Not always the kiss-of-death – Ken Loach’s The Wind That Shakes The Barley screened then and went on to win the prestigious Palme d’Or – you have to make a huge impact to be remembered in the minds of those all-important prize-giving jury members. And this year – with the second half of the festival showing films from such big-hitters as Terrence Malick, Lars von Trier and Pedro Almodóvar – is no exception.

Yet, it’s testament to the power of Lynne Ramsay’s We Need To Talk About Kevin that, at time of writing, we’re still talking about Kevin. With the awards ceremony to come on Sunday, there’s a good chance this brilliant, disturbing work will still be causing the jury collective nightmares. “With a film like this, you’re always going to have a controversial reaction,” acknowledges the Glasgow-born Ramsay, “but I feel good about the piece of work I made. There was a nice sense of satisfaction at the end of making the film.” As for coming here, “Cannes is a bonus.”

Wearing jeans, T-shirt and a straw hat, a barefooted Ramsay is looking relaxed – or perhaps relieved might be a better word – in a beachside restaurant on the Croisette the day after the world premiere.

She’s joined by her stars, Tilda Swinton and John C Reilly, and relative newcomer Ezra Miller, who plays Kevin, the twisted teenager who perpetrates a horrific act of Columbine-like violence at his high school. Adapted from the Booker Prize-winning novel by Lionel Shriver, it’s not hard to see why Ramsay feared a backlash. Instead, she’s been hit by five-star reviews wherever she looks.

“I just thought it was a very compelling story,” Ramsay notes. “I really like those Fifties melodramas – Douglas Sirk stuff, and All About Eve and Mildred Pierce. Really meaty subject matter. It’s sometimes dark subject matter, but you really want to see it. You want to go with it. So I was trying to make this accessible as well. It’s not an issue film. It’s a ‘What if?’ film. It’s a fantasy.”

Indeed, while it does deal with a teenage mass murderer, it’s much more about the difficulties a mother can experience when it comes to bonding with her child.

“It’s about a woman who doesn’t want to have a child who has a child, and still doesn’t want the child,” says Swinton. “And the child doesn’t seem to want to have been born to her. It’s about being a mother in really difficult circumstances. I think it speaks to something really not that exotic.

“There are many, many women who find themselves in these situations – and it’s a terrible taboo that nobody talks about.

There’s this whole assumption that the whole maternal instinct is going to kick in and what if it doesn’t? Sometimes it doesn’t.”

Set over 18 years, Swinton’s depiction of Kevin’s mother Eva must surely put her as a front-runner for the Best Actress prize come Sunday. Seen through her eyes, with the film flashing back and forth across events before and after Kevin’s murderous act, rarely has Swinton looked so beleaguered. “That’s that story,” says Swinton. “Where did I go wrong? What did I do? Was it anything to do with me?”

A mother of twins herself, was she troubled by the role? “It’s like looking under a rock or in your peripheral vision,” she says, after a pause. “It was quite exhausting to constantly try and look for that uncomfortable place.”

Still, compared to the trials Ramsay has endured in getting the project off the ground, Swinton’s few weeks spent last summer shooting in Connecticut were child’s play. “I don’t think anybody wanted to make it!” laughs Ramsay. “People thought it was way too dark.” Having first read Shriver’s novel before it was published, Ramsay was ready to make the film in 2008, only to see the £7.7 million-budget fall through. It meant stripping the script back – a process Swinton likens to grating cheese – to ensure that the film could be made for almost half the price.

Even then, it wasn’t plain sailing. “People were coming in off-piste on this, coming in having seen drafts that had been leaked out,” Ramsay sighs. “It was all nonsense. But I was determined to make the film. Basically, me and my husband [actor Rory Kinnear, credited with Ramsay as co-writer] went out to America and said ‘We’re not coming back until we’ve made it.’ We were meant to go for a week for casting and we stayed for six or seven months. It wasn’t going to happen otherwise.”

In the end, she crashed at the New York home of her esteemed cinematographer Seamus McGarvey (Atonement) and worked out every shot meticulously. “We planned the s*** out of it, because we had to,” says Ramsay. With no margin for error, this explains why she surrounded herself with such remarkable collaborators – from McGarvey to Radiohead guitarist Jonny Greenwood, who provides the score. Then again, Ramsay, now 41, hadn’t made a film since 2002’s Morvern Callar. So was she nervous going behind the camera again?

“It’s like riding a bike,” she smiles. “It really is. I love making movies – that’s where I’m in my element. The politics of making movies I hate. But the actual physicality of making movies – working with amazing people, being put into a situation where you have a gun to your head and you need to make a decision right away – I love that. The actual development is the thing that’s endless.”

Given that she saw her version of Alice Sebold’s The Lovely Bones grind to a halt – and then made by Peter Jackson – it’s easy to see why she feels a little bitter.

If there are scars from the Lovely Bones debacle, they seemed to have healed in time for Cannes. But while We Need To Talk About Kevin is the only British film in competition this year, Swinton is keen to set a distinction.

“I don’t think of Lynne as a British filmmaker at all,” she says. “First of all, she’s Scottish. And secondly, she’s an internationalist.” Considering this is Ramsay’s first American-set film, the actress has a point. Whether that will sway jury head Robert De Niro this weekend remains to be seen. But one thing’s for sure – he and the rest of the jury definitely still need to talk about Kevin.