Your group sits in a circle. One person
puts on a mask while the others ask questions, interviewing the character that
person is forming for the mask. The mask is then passed to the next person in
the circle who will attempt to play to same character, but maybe flesh it out a
little more and expand on the backstory as more questions are asked, while at
the same time maintaining continuity with what was said before. The mask keeps
going around, building momentum, until you have stabilized this single character
as a group, working out all the kinks and adjusting to assimilate and harmonize
with others interpretations. For greater challenge introduce more masks into
the circles, where those doing on the interviewing side have to interview in
character.
Once the character has bit fleshed out, the group can take the character on outings where they visit unique locations or engage in activities that relate to the fictional circumstances the character is going through. If you are in a public place you may have opportunities to interact with pedestrians. Remember that the members of the group are still passing the mask over the course of the trip.