I agree that this role is going to define someone and I'm not sure it will be in a good way. Just because something is massively popular as a book does not mean it will translate to the box office. There are lots of books that turn into bombs. Clive Cussler has sold millions of books but the one or two to make it to screen have sucked. I think the studio would love for this to be a franchise which means Jamie will be locked into playing the same character for a great deal of time and cement the character in the public's mind. That can be bad.
The Secretary was a one off and it can be said that Maggie Gyllenhaal's career has suffered since. Imagine if she had to play the character three or four more times over the course of a decade. Think she would be typecast?