1. They have no personal boundaries.
Stealing food off each other's plates is a given, of course, but it goes further than that. Rachel mixes up a drink and passes it around, and they don't bother to turn the glass or wipe it off before taking a sip. When Anderson's having a particularly long day, he pulls out his ever-present earplugs and uses the shoulder of whoever's closest (Keith, usually) as a pillow for a power nap. Once, without any warning, Stephen leaned over and plucked a hair from behind Jon's ear, and the scowl Jon gave him was all, "ow, that hurt," not, "what the hell do you think you're doing?!" None of them found it odd.
They do, however, decide it's best not to think too much about that time when they were spending a weekend at Anderson's house in the Hamptons and the boys (in various states of inebriation) all desperately needed to pee while Rachel was taking a shower in the only bathroom. It wasn't the first time one of them had made use of a toilet while another showered just feet away, but never four of them one right after the other. Rachel agreed to flush when she got out so long as whoever was last (Jon) closed the lid.
2. They go on trips.
"We should all take a vacation together this summer," Rachel said.
"We're all going to the Republican convention in August," Anderson pointed out.
"I know this is a difficult concept for you, Andy," Jon said, "but if you're there to work, it doesn't actually count as a vacation."
"Particularly if the GOP is there, too," Keith added.
"A real trip," Stephen said. "Somewhere we're not likely to get recognized. Or catch malaria." The last bit was aimed at Anderson, who shut his mouth on whatever destination he'd been about to suggest.
They wind up going to Brazil for two weeks, and Anderson is recognized anyway on the second day, causing the others to tease him mercilessly about being the most famous. He gets even a few days later, gliding through the ocean in his scuba gear effortlessly while the rest of them make fools of themselves.
3. They've got each other's backs.
When an interviewer tried to spin a comment Rachel made as her being critical of Anderson's decision to keep his personal life private, and it's subsequently picked up as the story-of-the-day by every media blog, she immediately called him.
"Anderson, I'm so sorry. I wasn't talking about you; you were never mentioned during that interview at all. That quote was taken completely out of context, and I'm going to post an explanation on-"
"Rachel, slow down," Anderson said. "What are you talking about?"
"You haven't seen my interview in The Guardian?"
"Well, I said something about gay people in the news coming out, and now everyone's intimating that I was talking about you, and I wanted to let you know that's not the case."
"Oh, is that all?" Anderson asked. "Rachel, don't worry about it. I never read anything about myself, anyway."
"I still don't like that they made it look like I was criticizing you, so I'm going to post a correction on my blog."
"If it'll make you feel better. But, y'know, even if I had read the interview and you hadn't called, I wouldn't believe whatever it was they were implying you said."
"Thanks," Rachel said.
4. They fight.
Anderson and Keith get into a minor snipe-fest at almost every meeting, and once a month or so they'll spend a few days not on speaking terms. It usually ends not when one of them apologizes, but when one can't resist making a snarky comment to the other, which launches them back into their cycle.
Keith and Rachel argue fiercely, but they almost never take it to a personal level. They can be squabbling one second and over it the next.
Jon and Anderson don't fight. Jon will take shots at CNN (which Anderson will laugh at and agree with) and occasionally at parts of 360 (which Anderson will agree with and apologize for). Keith often complains that if the same thing had come from him, it would've started a major argument, to which Jon says, "That's because Andy loves me more." Anderson nods, and Keith scowls.
Then there were the two weeks in the middle of the summer of 2010 when Jon and Stephen weren't speaking to each other. They both refused to divulge what the fight was about. The others were beginning to get seriously worried when it resolved as mysteriously as it had started: Stephen's hand on Jon's shoulder as they walked into the diner together, laughing over some joke. They sat down, waved off all questions, and slipped so seamlessly back into their usual rapport that Rachel later asked Keith if it was possible they had hallucinated the whole thing.
5. They love each other.
"I'd like to say something before we start eating," Anderson said as they sat around the dining room table in his new firehouse. "Though I've never been good at celebrating Thanksgiving, I've always liked the idea of setting aside one day to spend with family and count your blessings. I know you all have families of your own that you should probably be spending today with, but other than my mom -- who hasn't been big on holidays since my brother died -- you guys are it. You're my family. And... I'm thankful for you all."
There was a moment of silence, broken by Jon. "That was disgustingly sentimental, Cooper. You're gonna make me ruin my makeup."
"We're glad to be here," Rachel said with a smile.
Keith started to say something as well, but was interrupted by Stephen yelling, "Group hug!" and tackling him.