Kristen Stewart is an "awkward fidget", says Twilight writer Melissa Rosenberg.

Kristen is famed for playing Bella Swan in the hugely popular fantasy franchise. In the latest instalment of the series, The Twilight Saga: Breaking Dawn Part One, Bella is transformed from a human into a vampire.

Her on-screen husband Edward Cullen played by Robert Pattinson turns her into the bloody-thirsty character after a traumatic birth of their half-demon daughter sees her on the brink of death.

Melissa, who penned the big screen adaptation of the Stephenie Meyer's novel, says Kristen had to transform for the part.

"You know, Kristen and Bellasynonymouslyfidget. They're awkward," she told E! Online "and as a vampire, she's still. So you have to write toward that, like Remember to move!' Because as a vampire all that goes away."

In Part 1 of Breaking Dawn, audiences see Bella as a vampire for a few seconds before the credits roll. In the upcoming instalment - The Twilight Saga: Breaking Dawn Part Two, Kristen will be a vampire for the entire film.

Melissa is excited to see the 21-year-old star in action.

"I can't wait to see Kristen embody that. It's a great challenge for an actor," she added.

The Twilight Saga: Breaking Dawn Part Two is set for release next year.