You don't necessarily need to be extremely close in an emotionally connected kind of way, you just need to be very familiar with a person. There are people who I consider very good friends, but I wouldn't just tell them anything. Still, I can sit on the couch with them and just hang out. We could be having a conversation and suddenly it comes to a halt. Fine. We can just sit there and be comfortable. The point is that I know the person so well that I don't worry about what I'm supposed to be saying or doing. I know I'm not offending them by being silent. To me, comfort (usually) equals familiarity.