I had physical unusualness going for me (I'm fat and have big, puffy, curly hair) which was in stark contrast to the girl they wanted for Larken (Slender, with long, wavy hair.)
I also have a very brassy and untrained voice. It was very rough and comedic. Some directors wouldn't go for this, but mine really liked it. (I actually got better as the show went on, and she was like "No, bring it back to rougher voice!")
I think Fred is a very earnest and confident. I think she is aware of the fact that she is unusual and unprincesslike, but doesn't care. I think you have to play her bordering on obnoxious, but not so much that she annoys the audience. It's a fine line. They have to root for her.
This is my favorite musical, and my alltime favorite role to play so feel free to ask any questions.