Actually I'm not sure I agree with her decision to read the book. If the script went in a substantially different direction, then her initial reasoning was sound: she should base her performance only on the script's version of the character and not confuse things by considering an alternate slant. (Just as actors playing a character that's previously been played by someone else often choose not to watch the earlier performances, wishing to create their own distinct characterization rather than just imitating someone else.) A book is the creation of one person, but a film is the work of many. Her director might have a vision for his version of Nikki Heat that's substantially different from what's in the book, so if she draws too much on the book, it might clash with what the director wants from her.