There are 4 characters you will interact with during the story. At anytime when playing a level with each of these characters, you need to attempt to jump on their heads. Now you can't stand on their heads literally but you can come close for a split second. So you want to do your best to jump on their head and for it to count, the character has to SAY something to you. An example is Ludomir (Bear) will mention not being in the circus anymore or something similar. Once you hear the character say something, you're golden.