Keeping behavior professional/separating colleagues and friends is on the top of my list these days. I have recently had numerous conversations with younger attorneys in my firm about this - they think “oh, our managing attorneys are great, we can be open books with them! We can talk freely about our massive hangovers and romantic dalliances...” and I try to explain how much things are exaggerated and gossiped about. Not just in the office, but in the professional community. It is a sad but true state of professional work and all human interaction. Most people are simply not your friends and will go out of their way to tell stories about you. It is baffling.
I think also you have to be aware of people's incentives. This isn't a sailing club. If someone gets some inside information about you or your work, that can be used as ammo during political battles. A lot of the office "friends" are fishing for informational arbitrage. And even a fish can keep out of trouble if it keeps it's mouth shut.
