Hi! Current character performer here, though not Pooh height.
For how much you'll be characters- Well there are a few things that can happen. At first, you'll definitely be filling in where they need you. We get shuffled around a lot. Eventually, though, you might get put as one character in one location like, all the time. The Full Timers get bids, so they're in certain locations all the time when they work, and the CPs kind of take the empty bid lines. I guess we can also be put places if we're particularly good at certain characters, though I have no idea about that. You can talk to scheduling liaisons and they can try to put you as a certain character more often, too.
Justy is also right - you have to pass your training to be okay'd for your characters, and you can still get disapproved in things. One of my friends is disapproved in skinny boy characters like Max because she has a very girly figure. They're strict about autographs during training, but as long as you know it in general when you're out there you should be okay, as far as I know.
And if you're a princess or whatever, in WDW, you will also be doing Fur. All Face do Fur here, though that's not the case in
Disneyland.
If you have any more questions, feel free to PM me and I'll try to help!