She insisted that she was - with her siblings Ralph, 28, Rosie, 25, and Roxanna, 18 - always encouraged to perform. But "not just for the sake of being entertaining at Christmas parties or family get-togethers".
She added: "Our parents always brought home to us that, if we were artistic, it should always be something really good and special - not just for entertainment."
It was her parents' love of the theatre that first inspired Garai, who lives in West Kensington, to become an actress.
She said: "My mum took us to the theatre a lot and I remember being completely fascinated by it. I went to see The Tempest at the Bristol Old Vic with my dad when I was about eight, and I really laughed. For a child to laugh at a Shakespeare play at that age is incredible - it is incredible for an actor to achieve that."
Before long Garai was taking part in school plays and performing in competitions around Wiltshire. Her mother had already honed her young talent with speech and drama lessons when she was six.
Garai still remembers her teacher from those days, Mrs McKeown, whom she said "will always be an inspiration to me."
At 14 Garai, then a pupil at Stonar boarding school, joined the National Youth Theatre for an introductory course and continued to take part in school plays. She remembers Stonar as a "happy, jolly-hockeysticks place, where I spent my time skiving and making daisy chains".
It wasn't until Garai went to sixth form college at the City of London School for Girls that she appeared in her first professional production - the Bafta-winning Last Of The Blonde Bombshells, which her drama teacher at City had encouraged her to audition for.
Catherine Hill, one of Garai's best friends from Stonar, said: "She was really down to earth about her acting and never came out with stuff like 'I'm gonna be in Hollywood'.
"She was a complete natural at acting, and even the parents noticed how good she was and said she would make it one day. But I hope she doesn't go to Hollywood, as she's so lovely and natural. It would be a shame if she was spoilt by such a celebrity-looks orientated place."
