Honesty will be hard to come by, and your communication will get very confusing and difficult at times. You must be careful to be totally honest with each other, because any level of deceit in this friendship will take on larger proportions and become a big deal -- even if you've just told a little white lie to spare your pal's feelings. Passive-aggressiveness could also become a problem between you, especially on your friend's side; they might purposely mislead you when they're not aware of their feelings, or even leave out information just to watch you struggle without it.
Falseness in the opposite direction could occur as well: You might fake concern for their problems, but you'll have an ulterior motive. For best results, you must be careful from the start never to mislead each other or cover anything up. If you don't understand where your pal's coming from, just say so and move on from there.