In asking thisthis question I realized something that I never have before: my players aren't interested in each other's characters. How can I, as DM but also game session organizer and facilitator, help my players be interested in other players' characters?
Note: thisthis question is heavily related but from a player and that player is interested in other characters and wants to help them be fleshed out, so it's not quite what I'm looking for. I have tried some of the things mentioned in the answers like asking the players to describe (make something up) to answer a "Remember that one time?"-esque prompt.