Well, the MC and IC represent perspectives, and those perspectives are always like mirror images of each other. As soon as you can find the way that they are the same you can see how they differ on external vs. internal. (This is why those perspectives will always be based on Universe <-> Mind or Psychology <-> Physics.)
But for them to be proper mirror images of each other they need to be the same within their domains. Otherwise, the IC will not be able to suitably challenge the MC properly; his influence will glance off because it won't be entirely relevant to the MC.
Let's say you have an MC who is struggling to find a purpose in life. He feels he is supposed to do something important with his life, that he has an important future bigger than his corporate job, but he can't quite find it. Something seems wrong with the world so he uses his computer skills to hack into systems around the world, searching...
Meanwhile, the IC has a deep belief, an obsession, that he will be the one to find a new messiah to lead people out of slavery. And he has come to believe the MC is that messiah. This belief is so deep-rooted (subconscious) that he is willing to sacrifice his life for the MC, and this challenges the MC to see that maybe everything he was looking for is already within him.
Well, I'm sure you can tell what movie that's from. But can you see how those perspectives are really the same, except flipped on the axis of external vs. internal? One senses he is somehow special but is stuck searching for a purpose out in a world he can't quite grasp; the other is utterly convinced of his own purpose.
If you had a different match-up, keeping MC Future but going with say IC Memories, the IC's influence would be off target and the story would miss its mark. "You're telling me I can dodge bullets?" "No, I'm telling you that when you can remember your sister, you won't have to." Yuck.