One of the main complaints that go around about romances is that the reader doesn't buy into the relationship. People complain about "instalove," basically a modern term for Love At First Sight, which -- if you step out of the romance-glow and think about it -- really doesn't make any sense at all. Lust at first sight, sure. Infatuation (which feels exactly like love from the inside) at first sight, sure. But not actual love. Too many romance writers show their characters having boatloads of awesome sex and figure that's enough to communicate to the reader that they're In Love, but it doesn't actually work very well.
This applies to other kinds of relationships too. If your characters are friends, the reader wants to know why. What's the friendship based on? What holds it together? When these family members get together, do they really enjoy one another's company, or is it just duty visits and birthday cards?
Hugh MacLeod, a well-known cartoonist I'd never heard of before (thanks to Passive Guy for linking to him) talks a lot about social objects, the things (physical or conceptual or whatever) that link people together socially.
The Social Object, in a nutshell, is the reason two people are talking to each other, as opposed to talking to somebody else. Human beings are social animals. We like to socialize. But if [you] think about it, there needs to be a reason for it to happen in the first place. That reason, that "node" in the social network, is what we call the Social Object.
He goes on to give some examples:
Example A. You and your friend, Joe like to go bowling every Tuesday. The bowling is the Social Object.
Example B. You and your friend, Lee are huge Star Wars fans. You two invariably geek out about Darth Vader and X-Wing fighters every time you meet. Star Wars is the Social Object.
Example C. You’ve popped into your local bar for a drink after work. At the bar there’s some random dude, sending a text on this neat-looking cellphone. So you go up to him and ask him about the phone. The random dude just LOVES his new phone, so has no trouble with telling a stranger about his new phone for hours on end. Next thing you know, you two are hitting it off and you offer to buy him a beer. You spend the rest of the next hour geeking out about the new phone, till it’s time for you to leave and go meet your wife for dinner. The cellphone was the social object.
There are more, but you get the idea.
Hugh's mainly talking about social objects in the context of marketing, but I think they apply to characterization too. What are your protag's favorite social objects? What topic or thing will draw him or her into a discussion or activity, will make another person seem interesting or worth engaging, whether they agree or are having a good argument or are arch rivals? The subject of their interaction, whether it's something they like to geek out over, something they both support, or something they fight over bitterly, is a social object.
I've seen plenty of characters who don't seem to have many social objects, which can be particularly problematic if relationships (friendly, family, romance, business, whatever) are important to the story. (This means all romance books; the primaries in a romantic relationship had better have a few social objects in common -- aside from great boinking -- by the end of the story or there are plenty of readers who aren't going to believe in their HEA.)
Say Joe and Bob meet in college. They bond over their mutual hatred for the head of the French department, whose hard-ass views and rules, which she forces upon all the French teachers*, make it a lot harder for students to succeed. Joe and Bob also have a favorite eat/study table they share in the campus center, and they're both discus throwers on the college's track team. They're best buddies, yay.
What do they do after they graduate? The French department is now irrelevant, they're not eating or studying at their favorite table anymore (and don't have studying or assignments to collaborate on or help each other with), and discus isn't exactly a popular passtime for people who aren't active on competitive teams. What social objects do they share now? If the author's answer is "...?" then pretty soon it won't be believable that Joe and Bob have remained friends. I just started thinking about relationships in these terms, but it seems to me that a lack of social objects in common is probably one of the main reasons people who used to be close drift away and their friendship fades.
Ever notice how seldom you stay in touch with your old coworkers when you leave a job? That's a lack of social objects. If all you ever talked about was work related -- if "work and associated subjects" was your only social object in common -- then once you no longer work at the same place, you don't have any social objects to anchor your conversations and interests anymore.
Shared past experiences can be social objects -- that's often most of the glue that holds family members together despite different interests -- but it doesn't hold up under daily use. It's easy to anchor two or three conversations per year on twenty years' worth of shared experiences, but if you and your brother see each other once a week, you're both going to get tired of the repetition after a while, unless you have other things in common.
My brother and I visit two to four times a year, and talk on the phone maybe half a dozen times. We share news about our relationships, talk about working out (which he does a lot more than I do), he talks about his job and I talk about my writing, and we both talk about our mother. For a dozen or so conversations per year, we have enough social objects that hold both our interest. (Note that "our non-similar occupations" are a sort of reciprocal pair of social objects -- I'm not that interested in retail management and he's probably not that interested in writing and publishing, but up to a certain point each of us is interested in what the other is doing, because it's a brother/sister thing. That wouldn't work for a weekly or daily conversation, but for our level of communication we're both willing to swap listening time. There probably is (or if not, there should be) a special social object term for that kind of reciprocity between people who care about one another enough to be interested in Object X in a limited way only because it's an interest of the other person. [ponder]
But your characters who are best buddies from college need strong social objects in common post-graduation if they're going to believably be best buddies in the now of your story, ten or fifteen or twenty years later. They can only rehash college so many times before they're going to bore one another to death. And your romantic couple who got together while fleeing a deadly ninja mercenary squad who'd been hired to kill a pair of drug dealers who'd run off with their supplier's money, and for whom the ninjas (or possibly the supplier) mistook your couple are going to need some significant social objects to keep them together once the excitement of dodging death is over and done with. (And no, great boinking is not enough. Even reminiscing about that awesome time they did some really great boinking while hanging by their knees from the rafters of an abandoned warehouse, struggling to stay silent while the ninja hit squad searched for them forty feet below, isn't going to take them to their 20th anniversary, despite the fact that it's probably a pretty incredible memory. [cough])
Figuring out each major character's favorite social objects, and exactly which ones they have in common with their friends or others they're close to, and maybe which new ones they're going to be drawn to during the course of the story, sounds like a great exercise for establishing how each pair of characters connects with one another and what maintains their relationship, whatever it might be.
*Seriously, I only took one French class in college because the department head was an idiot and a martinet. She chose a textbook all the first-year classes had to use that emphasized memorizing dialogues over actually learning grammar, and refused to let the French teachers let the students keep corrected exam papers. Graded exams were passed out, students could ask questions, then the teacher collected and kept them. Everyone gets that language is cumulative, and that you might want to study previous exams when prepping for the next one, but the only way students could do that at my school was to go into their teacher's office during office hours and study there. Ummm, yeah. :/ I took one quarter of French and then ditched it, because the learning environment was ridiculous. [sigh]
Note, however, that I'm no longer in contact with any of my fellow students who hated it as much as I did. :P