The 'Harry Potter' actor said while he researched for the role much of his preparation was already done, reported Daily Mirror.
"I had two months, I think, to prepare the role. So I used all that time but I had never played a real person before, a historical figure," Radcliffe said.
"So, in a lot of ways it's a blessing because a lot of the work is already done for you. So you just have to go back and read it," he added.