WORKING FROM A GIVEN CHARACTER DESIGN
If you're working on any type of a production regardless of the size or number of people involved, you need to have a character design. Even if it's just one drawing on a napkin from a restaurant, it's the guide you will use to do all your other drawings in animation.
You need a character design in order to help you remember from one day to the next, just what your character is supposed to look like. It helps you keep them consistant. I've seen lots of examples where a character has started out at the beginning of an animation looking one way and then as the scene progresses, they strat to metamorphosize into something slightly different. They might just shrink a bit in overall size or worse, different body parts get longer or shorter. This tends to happen mostly with arms and legs. The worst case scenario is when the actual design of the character starts changing: they eyes get closer together, the length of the head changes or the ears shift. This is all with the same person doing the drawings... in just one scene!
If you're working in a studio and the project is much larger with multiple scenes, there will most likely be more than just one animator. This is where keeping your character "On Model" is absolutely necessary. Can you imagine what a cartoon of Bugs Bunny would look like if every animator decided to draw him in their own "personal style"? There would be radical changes to the design from one scene to the next and the feeling of consistancy would be lost.
The best compliment someone can give you is that you are an "artistic cameleon", able to draw in any different style without anyone being able to tell that it's not the work of the original designer. This is not to say that you can't have your own design style and do you own drawings your way, but when you are working on a group production, it must look like the work of one individual.
When you start working for a studio, unless you're the character designer, you must draw to the designs of the show. In some cases, even the character designer will have to draw in someone elses style depending on the requirements of the production.
Back in the 1980's, I worked in the layout department on the Care Bear's television show. The Carebears were designed in 1981 by Elena Kucharik who created the characters for a series of greeting cards. In 1983, Kenner turned the designs into plush toys, then in 1983 they were developed for an animated television series. At each stage, there was a designer who refined the characters slightly to meet the needs of the specific production. At Nelvana, there were a group of designers who created model sheets for each of the Bears and later for the Cousins and other related characters for the show. In the Layout Department we were all given a set of design sheets that we were told that we must follow exactly. We couldn't make them taller or fatter or modify them in any way.
Every production that you work on will be the same way. You have to adapt to the drawing style for that production. If you move on to a different production, you can't draw the new characters in the style of the show you just finished. After we were done with Care Bears, we started working on George Lucas' Droids series with R2D2 and C3PO. It would not have worked out too well if they were drawn in the same style as the Care Bears (although that is an interesting notion).
The best way to get ready for this now is to get a hold of some model sheets from some different productions and start trying to copy the design style. Start by tracing the drawings first. Then copy the actual poses on a separate sheet of paper by just looking at the originals and then finally, draw the characters in some original poses of your own, then check with the model sheets to see if you've kept them "on model". That's the exact same process you'd go through if you were working in a studio on a new production.