It's amazing to think how different this movie was when they were still in the early stages of making it (and how much better it is for the changes). Olaf was a very different character, a rejected prototype created by villainous Elsa for her snowman army, and because he wanted to prove he could be a good soldier he was trying to be tough and kind of mean a lot of the time. Whenever he came up, Jennifer Lee (writer/co-director), who had just been brought in, kept writing 'Kill the F'in Snowman!' in her notes. So they went back and thought about what snowmen actually represent and that's when he got his origin in Anna and Elsa's childhood.