Asked how it feels to see the glowing reviews and ecstatic audiences for her new continent-spanning love story, first-time filmmaker Celine Song leans forward, blushes a little and whispers: "awesome".

Song's "Past Lives", already a breakout hit at last month's US film festival Sundance, is having its global launch at the Berlinale cinema showcase in the German capital.

The reception for the semi-autobiographical romantic drama set in South Korea, Toronto and New York City has been rapturous, with industry bible The Hollywood Reporter already promising it will be "one of the best films of the year".

The movie tells the story of Nora, who begins an innocent romance with her best friend Hae Sung when they are both 12, then loses touch with him when she and her family emigrate to Canada.

They pick up their connection again via social media about 12 years later after Nora, played by Greta Lee of Netflix's "Russian Doll", moves to New York to pursue her dream of writing.

There, she falls in love with Arthur (John Magaro of "The Big Short"), a Jewish American, at an authors' retreat and eventually marries him.

Grown-up love

But Hae Sung (Teo Yoo of Park Chan-wook's "Decision to Leave") stuns Nora years later when he says he is flying from Seoul to New York to visit, having never quite gotten over the memory of her.

The tenderly climactic final scene left the Berlinale audience weeping and cheering, an experience Song called "overwhelming".

The 34-year-old playwright-turned-director told AFP that her debut feature was both specific to her life but also a "universal" story of grown-up love.

"I was thinking about the way it actually feels to have connections that span decades," even as we and those we care for change, she said.

"It was about how it felt to be a person who is doing an always extraordinary thing which is to love someone."

Song said she had her cast watch Richard Linklater's "Before" trilogy starring Ethan Hawke and Julie Delpy to pick up lessons about depicting romantic longing that evolves over years.

Rather than building to an explosive confrontation between "man-children", so common in US movies, Song said she wanted to show a mature kind of intimacy.

"A lot of drama can happen if everybody decides they're going to behave like children," she said.

"But I know so many people who are in my life who make a decision every day to be adults."

In the film, "we're watching these three people who are very connected to their childhood" but still opt to "care for each other and to respect each other and be there for each other".

Korean boom

Song said she was "proud" to join a boom in South Korean entertainment, even if her film had little to do with the bloody spectacles of Oscar winner "Parasite" or Netflix's hit "Squid Game".

"The exciting part is that it's possible for 'Past Lives' be so different (from those ultra-violent stories) and for it to still have an audience who feels that it is global," she said.

Song "ended up being a kind of bridge to different kinds of filmmaking cultures," she said of bringing her New York crew to Seoul, where her rejection of classic storyboarding and focus on rehearsal seemed foreign.

"Past Lives" was leading critics' polls among 19 contenders at the festival ahead of an awards ceremony Saturday.

Asked what it would mean to claim the Berlinale's Golden Bear top prize from jury president Kristen Stewart, Song broke into a wide grin.

"At the end of the day we're a little movie that wants to be seen," she said. The Golden Bear would "mean that more people will want to see it and that's what I dream of".