Radcliffe, who is known to most movie-goers as the boy wizard Potter, says it felt "very natural" to move away from that character and start the next phase of his career.
The 22-year-old plays a widowed father and lawyer in the film adaptation of the Susan Hill's gothic ghost story, one of the longest-running plays in London's West End.
Nonetheless, many of Radcliffe's fans lining the red carpet at London's Royal Festival Hall on Tuesday greeted him with Potter books and other memorabilia to be signed.
Radcliffe attended the premiere with his co-star, British actress Liz White, who plays the title role.
