Gellar will play the roles of identical twins in the forthcoming drama series, which will premiere in the autumn in the US.
The 34-year-old has admitted her new role in the show is challenging.
She told Entertainment Weekly: 'With Buffy, I got to do so much. So what do you do next? What do you do that an audience hasn't seen you do?
'Bringing to life two characters and making them different, but at the same time interesting, and showing the similarities is definitely a challenge I've never done.'
The show follows the story of a young woman who is on the run from the mob and hides out by inhabiting the life of her twin sister, until she finds out that her twin's life has a bounty on it as well.