"IT'S nuts, isn't it?" Sarah Bolger said.

The 18-year-old Irish actress is talking about the scene where she is fending off a group of invisible goblins in The Spiderwick Chronicles.

"I was fighting nothing – there was just air around me," she said.

"You don't expect it to be like that – there is a point were you're swinging a sword at nothing thinking 'what am I doing?'"

As Molly Grace, she is the older sister of twins Jared and Simon Grace in the new fantasy film based on the children's novels by Tony DiTerlizzi and Holly Black.

"I loved the books," she said, "I'd read them all before I knew about the film," Sarah said.

In a Dublin accent which seems a million miles away from her portrayal of a cynical New York teenager in the movie, Sarah is bubbly and upbeat.

"I always loved performing, I wasn't sure if I wanted to be an actress but I knew I wanted to perform," she said.

It was through her younger sister, Emma, that Sarah got her break with an award-winning role in Jim Sheridan's In America.

The pair played siblings in the critically acclaimed film, which told the story of an Irish family adjusting to their new life in the United States.

This new film, however, is not quite as grounded in reality.

"There's a lot of stuff in Spiderwick which is quite unusual," Sarah said.

"There was the twin thing (Freddie Highmore plays Jared and Simon Grace), the characters who aren't there and, for me, there was having
to put on an American accent."

And it meant a host of new experiences for Sarah.

"This was the first movie I've ever done like this – it's unreal,"
she said, "But we were in good hands."

She had particular praise for special effects guru Phil Tippett – who worked on films such as Star Wars and Jurassic Park – for keeping her right.

The Spiderwick Chronicles were shot in Montreal over a four-month period.

"This was the longest I've been away from home," Sarah said.

But, because of the special effects used in the film, it was only when Sarah saw the finished article that she was able to appreciate the size
and scale of the movie.

"It was unbelievable to see it when it was done," she said.

"They had a storyboard – and I was able to look at that – but we never saw any of the creatures or the characters until the premier."

The modest actress is, however, not going to talk up her contribution to the piece.

"I can never look at a film and say I did a good job," Sarah said.

"I'm proud to be part of it, and it was great fun to make, but I'm not going to say I was the best actress in the world."

After turning 18 at the end of last month, Sarah said that she is going to build on an already impressive CV continue her career in acting –
despite parental concerns.

"My mum would love me to go off and be a doctor or something, but I don't think it's going to happen," she jokes.

The Spiderwick Chronicles opens in cinemas on Friday.