In our discussion, the population of the discussion changed significantly over the course of an hour. At first there were three of us present, all mothers; then two more people joined us, not mothers; then another mother joined us, one in a non-traditional family; then a father joined us. Each time someone new joined the conversation the tenor of the talk and the topics we discussed changed significantly.
If I were to characterize the way that the talk changed, it would be something like this.
When it was just the three mothers talking, our focus was mostly on the particular differences between our various experiences (and question of how often people ignore such variation in fiction). Lots of stuff was sort of assumed as common ground and therefore not mentioned.
When the non-mothers joined us, all of a sudden all that common ground became open to conversation because there were people present who did not share that experience with us. So variation was still a focus, but the range of topics discussed actually broadened - and I would remark that the mothers among us also varied on some of the variables upon which we'd previously assumed commonality. We progressed in our topic from pregnancy to babies and childcare, but a lot of it still focused on the question of realism - how to portray pregnancy and babies in a realistic way by expanding our knowledge of people's real experiences.
When the mother in a non-traditional family joined us, a lot more topics opened up - not just the basics of pregnancy and parenting, but further economic variables became important, and we started talking about how the experience of parenting had changed our lives (something that hadn't come up before). Each of us had had a very different experience.
When the father joined us, the discussion changed again because he had a particular fiction project he wanted to get inspiration on, and that changed our talk entirely. Instead of talking about a general range of experience, we started talking about a particular person's situation in a particular world where the social and economic conditions - and the person's story conflict - were very different and could be expected to interact with the problems of parenting in a very specific way. For a mother on the run with an infant, what would be the tricky issues? Suddenly we got very specific about diapering and breastfeeding and bonding between mother and child, changes in expectations about what would be taken care of by the mother and what would be handled by her support system, and how the disappearance of that support system had changed things.
I take two things out of this that may be of use for writers. First, for worldbuilding in general: the worldbuilding phase itself is useful, but you'll be doing a lot more worldbuilding and you can learn a lot better stuff when you're working with specific situations and conflicts of the characters in a story. Second, be aware that the dialogue and conversations you write will be drastically different depending on who is involved. The arrival of a new character will most likely completely change the tenor of the talk.
The following factors (at least) will be important to what kind of changes occur:
- each person's amount of personal knowledge and experience on the topic
- each person's understanding of how much knowledge and experience he/she shares with other people in the discussion
- each person's personal motivation, i.e. what specific information he/she hopes to get from the talk