This all started because Joe Madureira (!!!) reached out to us about a crossover. We liked the idea, so I reached out a bunch of other indies and they all agreed.
We usually approached them with a character in mind, but ultimately who went in was up to them. Sometimes it was based on who was the most recognizable, but in the case of Calibretto, we wanted someone that would really stand out from the rest of the cast.
And we tried to get some kind of balance in the roster, i.e. a grappler instead of a straight-up melee guy, people that have abilities in their games that could be "mage-y" in Indivisible's battle system, etc.