The supporting characters typically carry less story/plot weight - so you can be more broad and pushed with them. Supporting characters also take up less of the film's screen time. A short is a great opportunity for supporting characters to shine.
When you develop an entire feature length film from scratch the challenge is developing an entire feature length film from scratch! - the world and all of it's characters need to be created. There is no story/plot - all you have is a blank sheet of paper.
We really throw ourselves into our work and the details from your life show up from time to time in the finished film. Our personal experiences really help to bring shape to the movies we make at Disney.
Developing a full length feature is much longer process than developing a short. With features you're typically dealing with more characters, plot, emotion, story arc, etc. - a short is the same only much... shorter!
Animation story boarding works differently than live action story boarding. The story crew along with a writer really does shape and create the film - the world and it's characters. We meet almost every day and brainstorm the plot of the film. It's a highly collaborative process - and we continue to improve the story until we literally run out of time.