Williams has previously starred in Kiwi productions Shortland Street and Almighty Johnsons, and cult American show Spartacus.
12 Monkeys, which stars Amanda Schull and Aaron Stanford, charts the journey of a time traveller who's come back from the future on a mission to eliminate the source of a deadly plague that nearly destroyed the human race.
Williams was unable to divulge details of her character, other than to say she was "strong, but incredibly vulnerable".
She said the role was a perfect fit.
"It's kind of like dating, you either have an immediate chemistry with the character or you don't."
Williams said it was a "good credit" to have, but attributed success for the most part to perseverance and a bit of luck.
The brunette had been in Los Angeles auditioning for another show - but failed to make the cut when the 12 Monkeys audition noticed cropped up.
Initially producers were looking for a blonde, but her agents pushed to get her an audition spot.
Shooting for the series took place in Toronto and involved long cold nights in the Canadian forest.
"We were in the forest, in the dark, with snow falling around."
Her character was involved in a series of fight scenes, where she had to learn a few martial arts moves and use weaponry.
"It was tough," she admitted. "But I got a pretty epic fight scene."
After spending three months in Los Angeles and being flown back and forth from the shoot location on Toronto she's back in New Zealand.
She's filling in the time working on a local film, as well as teaching acting classes around Auckland.
Williams admitted it could be tough at times, going from one "incredible job" into the unknown.
"That's tough, because you go from such a high, to such a chasm."
