IT'S one thing to star in a film opposite a screen legend like Diane Keaton.

It's another to share a character with her.

Australian actress Claire van der Boom does just that in the comedy drama 5 Flights Up.

Based on the novel Heroic Measures, the film follows long-time married couple Alex and Ruth Carver (Keaton and Morgan Freeman) as they become overwhelmed by personal and real estate issues when they plan to sell their Brooklyn apartment.

Van der Boom plays Ruth in her younger years in a series of flashbacks revealing how the couple met and fell in love in the 1960s.

"I watched all of Diane's films and read her autobiography," she told APN.

"No matter what she does, she has a through line with her personality which is vibrant, smart, catchy.

"Her nervous system is a little heightened.

"You take these traits that are in The Godfather or Annie Hall and you just have to capture the spirit rather than try to mimic her."

In the big-screen adaptation of author Jill Ciment's novel, the central relationship between Alex and Ruth was changed from a socially taboo age gap to an interracial marriage.

"Even though it is set however many years ago, there are human issues (in the film) that are timeless," van der Boom said.

"What's going on at the moment with gay marriage and interracial relationships, it's important to me and it's upsetting to me.

"There's a lot of work to be done in a lot of social areas.

"It was not hard for me to tap into what it would be like to be someone who has a family that doesn't support you because of where your love lays."

Van der Boom, best known to Australian audiences for her roles in Rush and Love My Way, is currently in Atlanta filming Game of Silence, a new drama for the NBC network.

She has been based in the US since landing a regular recurring role on the TV series Hawaii Five-O opposite fellow Aussie Alex O'Loughlin five years ago.

She also stars in the upcoming films Dear Eleanor, Battlecreek and A Year and Change.

*5 Flights Up opens in limited cinemas on Thursday, August 20.