Often you two will be on a different plane, mentally and emotionally. Your very different ways of looking at the world and any given situation in it will inevitably lead to misunderstandings between you. There will be a struggle on both your parts: You'll always be trying to get your friend to focus and communicate in a clear, logical, straightforward way, and they'll resist, fighting to remain the person that they are.
You'll probably misread their cues, as well, or just not trust them. It's not that they'll be untruthful, either. They just have a different way of being in a friendship and expressing themselves that will be unrecognizable to you. You should both try to remember that you're simply different people, but that doesn't mean your pal is dishonest, flaky or anything else of the sort. One thing is sure: You'll have to work to understand each other.