So after a talk with Phil, we agreed that an Iceberg is an interesting metaphor as it is very relevant to the "Smile" in terms of having a clear side and hidden side.
To make this metaphor more interesting I have thought of ways to show that the hidden side of the Smile is the brain and the face illusion idea came up.
I find it interesting in a way that it tricks the audience to believe that the first character's head is another's face and vice versa.
I will be soon uploading a storyboard which will make the idea clearer.
Another idea was using a house as a character to show that the bottom bit (Attic) is the brain as it is place where past memories are hidden, and the rest of the house (minus the attic) is what you normally see and interact with, as no one want you to look at their attic.
Below is an idea about using an undercover agent drawing to show how certain things could be hidden under a hat, taking the example of Inspector gadget.
I really like the first idea and will take it further to a storyboard.