The Man from U.N.C.L.E. star has played medical examiner Dr. Donald 'Ducky' Mallard since the series began 10 years ago, and the job spark an interest in pathology, so he now studies the subject on the side.
He tells Britain's Daily Record newspaper, "Very early on in my career, I realised there would be many days when I wouldn't be working so I should find something to do, such as woodwork, painting, trying to play golf, or learning languages. For the last 10 years, I've been studying pathology. I've got about six feet of books on death on shelves in my house, going back to the earliest Chinese studies on autopsies."
McCallum even attends real autopsies to perfect his portrayal of a coroner.
He adds, "(Co-star) Mark Harmon is always telling me I am obsessed and I guess I am to a certain extent. I've been down to the Medical Examiner's office in Los Angeles with coroner Craig Harvey and I've managed to not just watch autopsies through the glass, but actually stand in with him while he is doing one... I just want to get it right. I get a lot of letters from real pathologists thanking me for representing their profession with a degree of accuracy."