There's a lot of literature online about the importance of preparing for an audition. And it's true that it's' better to be prepared, and be familiar with your sides, and have a fully-formed idea of your character before stepping into a casting session. But there's also a slight possibility that if an actor focuses too much on memorizing lines, and perhaps doesn't but enough emphasis on allowing themselves to be lost in a character, their audition may be perceived as a route recitation of lines, rather than an emotionally-evocative performance.
In essence preparation never hurts, but it's important that the process you employ to help prepare for an audition doesn't detract from any spontaneous magic that could potentially emerge when you're under the skeptical eye of a soft-box light in a casting room.
Filmmakers also face in a dilemma in assessing talent between actors that are wholly prepared (but not spontaneous), and actors that are too improvisational. Top filmmakers that have published blogs typically encourage one another to respectfully help an actor break through a series of prepared lines, and challenge them to get slightly out of their comfort zone. As a filmmaker, if you sense that an actor cared enough about a role to over-prepare for an audition, then it's OK to throw them a few curve calls to see if they can still hit it out of the park.
So essentially preparation is a good thing- but don't loose any magic in the casting room!
I'm busy working on my blog posts. Watch this space!