I don't over-sing anymore, which I used to suffer from terribly because I couldn't hear myself.
AMC has a track record for finding actors who have been working actors but not names yet and casting them.
But things such as 'Harry Potter', all I can do is shape my character, seek the director's approval on that, and basically take it from there. Professor Flitwick in 'Harry Potter', I kind of defined how I saw him from reading the book, and luckily that matched up with the director's vision.

