Writing is like travelling. You never know who you might meet, and you damn sure better keep your head up or you might miss something. (Or your plot might get hijacked by a new idea.)
In order to write my next book I've got a lot of research to do. I plan a large scale world which has several (five or more prime and several mentioned) religious-based cultures. To that end, I've got to read large sections of the Bible. I'm going to read the Koran and study up on Buddhism. Not too tough around here, we've got Naropa nearby. That one is a bit intimidating, though, because I'm scared I might actually quite like and agree with Buddhism. I want to take a look at Native American beliefs, and Catholism, and African beliefs. I'm mostly interested in how those religions influence and inform the cultures, but I think the hardest part will be making them natural--blemished and shining at once.
I'm also trying to discern cultural anomolies based on language. Of course, while I don't speak any other languages, I've always been intensely fascinated by them. You can't understand a culture without knowing its language. Sometimes that language is even your native tongue, but if it's spoken with so much as a different accent then you've collided with a new culture. (Like people from the East Coast insist that they have their own culture, and they do! I just don't agree with the "superior" part.) I've got two accents which I can produce at will: Chicago and Colorado. Very different accents, very different cultures.
I like to listen to people talk, especially when they work together. I like how they relate in abbreviations. I like their knowing nods and rolled eyes. I like to eavesdrop on convos in bars, because the participants so often are not connecting, and I like to watch people try to overcome that clumsiness. I like to watch twins, who seem to know each other's personal space as well as their own. I like to watch my kids take each other for granted. I like to watch guys play basketball at the gym, because they communicate by touch. Most sports are interesting that way: they each have their own body language. Like in tennis, it's the player walking across the court, head down, breathing hard. In horseback riding it's a subtle caution around the animal, while always keeping fear at bay. I like to see couples argue over something stupid and resolve it without really speaking to it. Culture really is just one giant relationship.
Everything is fodder for ideas, but, funny enough, research always skews my original ideas and twists my plotlines. Like I just discovered the Anta, which is a secret religious sect in the British Isles. Not much is known about them, but I must do more research, because it sounds a bit like the "people" in my series. This will most definitely influence my treatment of plot and characters, and those books are in 1/2 way revised form. (Mostly they're so poorly written that even I sometimes don't know what the hell I'm trying to say. They'll get there someday, though.)
And then you get everything nailed down and the characters get to talking to each ther. They influence events, just like real people do in the real world, and fuck it all up until I hardly recognize my own book anymore.