The Pan's Labyrinth helmer's upcoming retelling of the classic fairy tale will star Watson, with del Toro writing, directing and producing.
"We met a week or two ago, and we started working on the script, and it's going to go next summer," Watson told MTV, before admitting that she had been nervous because of the amount of time that had passed since her casting.
"Obviously I've been doing a lot of other work, and I was nervous. I was like, 'Is this going to go away?' And Guillermo is so psyched about it, and he's been thinking about it so much."
Watson also said that she was eager to begin shooting the film after looking at del Toro's sketchbook of images.
"I was like, 'Ah, this is killing me!' I'm very excited."
Del Toro completed production on alien invasion drama Pacific Rim earlier this year, which is now being post-converted into 3D.
Watch a trailer for Watson's upcoming film The Perks of Being a Wallflower below: