Trivia: The movie makers really wanted the audience to believe that Michael Rooker was the killer, rather than Leland Orser. So much so that Rooker actually played the killer in almost all the scenes where the killer wears a mask. When you see the eyes of the killer through the mask the eyes are Rooker's.
Trivia: The Director, Phillip Noyce, makes a cameo appearance as the man in the bookstore.
Answer: The film makers changed the name of Amelia Sachs because it sounded too much like 'sex'. I have not been able to discover for certain why they made Thom into a woman, but possibly this had to do with Thom's homosexuality. It could be viewed as controversial that Rhyme's nurse is a gay man, and could possibly also bring Rhyme's own sexuality into question. To make the nurse female would avoid all such issues.
Twotall