It's all in the shoes by John

Christian Shepard wore Jack's shoes in transit to the island. We see one of his shoes dangling from a tree in the pilot.
In the mobisode, we see him barefoot in the jungle telling Vincent to find Jack.
Locke wore Jack's father's shoes in transit to the island. He took them off when they crossed from the smaller island to the larger one.

DOES MIB/Esau inhabit the form once the protective shoes are off?

They seemed to make a big deal about Locke taking off his shoes while in the little boat.

The only problem with my theory is that Hurley saw Christian Shepard in the cabin wearing the shoes. But there was another person who popped up right at the window and freaked Hurley out. Who was the other person? Or was that MIB who just did a jump to the window?
Not a very developed theory, but there seems to be something about the shoes. This is to get discussion going, not to say I think I have any right answers.