You'll have to force yourselves to be completely honest and clear with each other; otherwise, your friendship will become entirely too muddied. That means no telling lies or fibs to each other, even 'harmless' little white lies. Passive-aggressiveness could 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 go on from there.