Atlanta-born Moretz was bitten by the acting bug at the age of 5 when she stumbled upon a television airing of the 1961 classic film starring Audrey Hepburn, Breakfast at Tiffanys.
"Very simply, Audrey Hepburn made me smile," Moretz said.
"She made me be happy and I realised that's what I'd like to do -- make people feel, evoke emotion and evoke real life experiences."
Moretz, now 17, is the teen actress most Hollywood studios, directors and producers go to first if they have a movie requiring a youngster with the acting chops of a veteran.
Moretz's first major acting role was as a 7-year-old opposite Australian actor Simon Baker in his TV series The Guardian in 2004, a year later she was in the re-make of The Amytiville Horror but hit the big time in 2010 with her expletive-laden role as Hit-Girl in the cult superhero film, Kick-Ass.
If I Stay, adapted from Gayle Forman's best-selling novel, is considered the biggest challenge of Moretz's career.
Mia is the straight-laced, musically-gifted daughter of hippy parents and her life is smashed by tragedy when she, along with her mother, father and little brother, are in a car crash.
As Mia lies unconscious at the accident scene, she has an out-of-body experience.
It continues when she is in a coma in hospital.
Mia, with her rocker boyfriend and grandparents holding a bedside vigil in intensive care, is eventually faced with a decision -- to live or die.
"A mix of gratitude and relief," Forman replies when asked how she felt when Moretz agreed to play Mia.
If I Stay was first published in 2009 and in 2010 it was announced it would be adapted into a film, with Australian actress Emily Browning one of the frontrunners to play Mia.
Moretz was another candidate, but then just 13, was considered too young.
There was a delay in filming, several directors, including Twilight's Catherine Hardwicke, came and went, and by the time the production was back on track in early 2013 Moretz was old enough to be Mia.
"Chloe disappears into her roles," Forman said.
"I was just telling her I was in a hotel somewhere the other night clicking through channels and I settled on a movie.
"It took me 30 minutes to realise Chloe was in the movie.
"She just disappears in her roles."
One of the key scenes in If I Stay is a flashback to a nervous Mia walking on stage in a large theatre in San Francisco to audition for a scholarship to New York's pre-eminent performing arts conservatory, Juilliard School.
Moretz could relate to Hall's nerves.
The actress faced a similar situation in 2010 when she auditioned for Martin Scorsese in London for one of the lead roles in his multiple Oscar-winning 3D adventure film, Hugo.
"I walked into the room and Martin Scorsese was right there in front of me," Moretz said.
"That was a moment where I felt the nerves.
"I was just like Mia -- being on a stage and being right there, live raw and real and attempting to do my best."
App users: Tap here to view the trailer for If I Stay
Just like Mia, Moretz let her talent take over her body, although there was one hiccup.
Scorsese was after an English actress.
"I lied to him in that room," she said.
"He asked me if I was British and I said, 'Yes, I'm from Cambridge' and I thought he fully believed me until the end when he said 'I know you're American'."
Who: Chloe Grace Moretz
What: If I Stay
Where and when: In cinemas on Thursday, August 28.
- AAP
