It's quite common for the personality of a character to jump around a lot during their first 50 posts, it usually takes a while to get them down :P We see that in tabletop games too! People will often fumble around in random directions with their character before they eventually solidify on a single theme or personality. Expecting people to know exactly what their personality is going to be like at the beginning of a RP before they've even tried their character in action with the other players is a bit optimistic methinks, and might run the risk of being unnecessarily constraining ^_^