A few questions

Hey everybody.
I have a few quick questions now that Given has been introduced.
How are posts involving multiple people's characters done? Do I simply make an educated guess as to what another persons character will say? Do I email or message them and write up the post slowly, but surely as a back and forth? I've heard of doing both, but I am unsure how it is done here.
Sorry if this seems weird, but I'm really not sure.

~Stark