Kristen Stewart was just 17 years old when she was cast as Bella Swan in the first Twilight movie back 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 but I think we all kind of imagined it was fairly cult-ish.”

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 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 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 mean I started when I was 10 years old so I definitely wouldn’t trade anything.”

As the saga comes to an end, the publicity-shy Stewart is determined to savour every moment.

“You get slightly desensitised when you know there’s another one coming up, whereas today it genuinely feels like the last day of school, like you’re giddy and it’s hard for me to focus entirely because I’m excited,” she says.

And be in no doubt that this really is the end of the vampire love story.

“I don’t really know what we’d do beyond it. It feels ... conclusive!”

* Robert Pattinson’s one of the biggest heart-throbs in the world thanks to his portrayal of the beautiful vampire Edward Cullen, but the actor behind the fanged poster boy has remained as self-deprecating as he was at the start of the journey.

The 26-year-old is “pretty excited” that the release of the final instalment is imminent.

“It’s really good that we didn’t get stopped in the middle of the series. We managed to get to the end and I didn’t get fired!” he laughs.

He recalls his first outing as Edward.

“I was trying so hard on the first one. I’d never done an American movie before, I’d barely done anything before, and I kind of wanted to take it so seriously and was so determined to prove myself. Occasionally my face went totally out of control,” laughs the London-born actor.

Despite a role as the doomed star pupil Cedric Diggory in the fourth Harry Potter movie, Pattinson didn’t think he stood a chance of being cast as Edward.

“Someone had sent me the book six months before and I’d read it and I was like, ‘I’m never going to get this part, there’s no point in me doing an audition tape.” So he didn’t.

But then a while later he made his way to LA for a different role and “went in (or the Twilight audition) for the hell of it”.

Despite the mind-boggling hoopla that surrounds him, he’s never regretted his decision to take part in the franchise.

“It’s impossible to regret anything because it’s random how things work out,” he says. “You don’t want people to stop seeing your movies. You learn to live with whatever circumstances you’re given basically.”

He can’t recall his strangest fan experience because “I’ve got a very high threshold for weirdness”, he explains.

“Someone could come up and show me a tattoo of my face on their face and I’d be like, ‘It suits you!”’

Despite the global worship and a net worth that runs into many millions, Pattinson doesn’t think the Twilight experience has changed him.

“I think I’ve remained the same as I was when I was 22, like in a bad way,” he chuckles.

And asked what he thinks he’d be doing if he’d never gone along to that audition, he says: “I don’t know. If I’d continued living the way I was living in London before I got this I’d probably be 350 pounds and a drug addict!”

* The Twilight Saga: Breaking Dawn Part 2 is released on Friday.