To play as mickey is very simple, go to the mulan stage (the first time you go)then complete everything until you arrive at Shang Yu, the boss, lose to Shang Yu, and after you die it will give you the chose up giving up or continuing to fight, press, "never give up" and you'll playing as mickey and everyone else will be dead like sora. You can take down about half of Shang Yu's health and then it'll make you use the move healing light and revive sora, then you'll pick up from where you left offf with mickey.