The British actor plays the role of Arthur Kipps in the movie about a lawyer who visits a haunted house, and he admits he was jumpy when he read through the screenplay, which was a good indicator to him that audiences would be scared.
He said: "It was a very compelling, frightening story. It frightened me when I read it, which I took as a pretty good indicator that it would frighten people when it was filmed.
"And when I met the director James [Watkins] I became even more excited to be a part of it because he was this smart, young, ambitious, hungry director. And that's always wonderful to work with."
The 'Harry Potter' star also found reading the script particularly easy as it was more like a "novel" than what he normally has to see.
He added to Bloody Disgusting: "The script was so good for this. Normally if there's a script that has a lot of stage direction it can be hard work wading through it.
"But this just read like a novel. The stage direction was so compelling and beautifully written and so clear, and so visual actually, that just reading it you had a sense of what the film was going to look like."