A couple people in yesterday's comments noticed that Kaspar was slowly changing- evolving or devolving, depending on how you look at it.
I think change can be good and bad, depending on what changes. Old characters, like Bugs Bunny changed all the time.
He changed for the good for a few years, and then kind of devolved into becoming an inactive logo.
I think you should let things change naturally, as you add shades to your characters, but it's god to go back every once in awhile and check to see if the core has changed too much-what made the character appealing and unique in the first place.
So I went back and looked at my presentation drawings and ate a bunch of bacon (which helps me think) as I drew the original things I noticed that had changed. His arms had gotten longer, and his face bigger, and his ears gravitated to the middle of his head. I don't think those elements are essential to the core of Kaspar, but it's fun to try different things. If you purposely try not to ever change anything, then the characters quickly become flat and stale.
I believe in drawing by feel, rather than slavishly following model sheets. When you are working with another person's characters though, you should be very analytic of the drawings he or she gives you and get it accurate for awhile, until you yourself understand the core, and then you can start letting your own feel and style peep through to help shade the personalities and design. Well, if the director lets you, that is.