The 3D film, which will use puppets and stop-motion animation, was announced at the Cannes Film Festival this week.
Del Toro, who will direct and produce, stated that Radcliffe has approached him to "get involved in the project', according to The Hollywood Reporter.
He then went on to explain the film's complex development process – he was at one stage planning to hand over directing duties and only produce, but ultimately decided that he was too involved.
"When we originally talked about this project I was going to direct, but then I said I would [just] executive produce it," he said.
"But everything with the production discussions became more and more specific and I began to realise I was getting more and more involved."
After confirming Radcliffe's interest, del Toro named Tom Waits and Christopher Walken as entries on his "wish list" for the voice cast.
The film is tentatively due for release in 2014.
Radcliffe was recently cast in a new Sky Arts comedy drama alongside Jon Hamm.
Watch Digital Spy's interview with Radcliffe for The Woman In Black below: