Why does MIKE look like Phillip Gerard?

I'm in the middle of The Return right now (episode 7, no spoilers past there please) and can't figure out why MIKE looks just like Gerard. BOB, the black lodge entity, has a clearly separate appearance from the humans he inhabits, and the Giant looks different from his host/human form. It's also made clear that Gerard is a separate person from the entity of MIKE, and presumably MIKE existed before him. Does anyone have any theories about this? Or was it just to avoid confusion/hiring more actors since Gerard's character only exists to be MIKE's host?