The final film in The Twilight Saga, Breaking Dawn: Part 2, hit cinemas earlier this month.
Kristen was just 17 when she was cast as Bella Swan in the first Twilight movie in 2007.
Back then no one knew what a global phenomenon the movies would turn out to be.
"We didn't even know a sequel was possible. It genuinely wasn't the goal," says 22-year-old Stewart.
"Twilight definitely had a following (thanks to Stephenie Meyer's novels) but I think we all kind of imagined it was fairly cult-ish. Then it suddenly took off like wildfire."
In this fifth and final instalment, Bella, who married vampire Edward in the last film, awakens transformed.
She's now a mother and finally a vampire but her daughter Renesmee (who's half-human, half-vampire) ignites forces that threaten to destroy the vampire clan and the destiny of her werewolf best friend, Jacob Black.
"Most kids that are attracted to the idea of being an actor are just like little attention-seeking weirdos, I find," says Stewart, who herself was a child star.
"But Mackenzie (Foy, who plays Renesmee) was so solid and steady. Nothing was ever about her, it was always about being a part of the group and wanting to help out and that's how I was when I was a kid."
Although constantly chased by paparazzi and hounded by the films' fans (aka Twihards), Stewart says she's never regretted signing up.
"There have been some heavy moments obviously but I would trade nothing," says the Los Angeles-born actress who endures ongoing scrutiny over her on-off relationship with co-star Robert Pattinson.
"It's such a strange job; you need to be a little crazy, you need to be a little obsessive. But how often do people find what really genuinely challenges them in life at this age?" she says.
"I get so much more time to do what I love than most people.
"I started when I was 10-years-old so I definitely wouldn't trade anything."