I haven't played enough campaigns or campaigns for long enough to really learn about my party's other PCs in great depth, unfortunately, longest campaign I've played was about ten sessions and that's really only scratching the surface.
Mechanically, I enjoyed playing with a PC who was a Jekyll and Hyde character where the player handed over control of their current state to the DM. The alignment of each was polar opposites, and the other PCs had no cue (that we picked up on), which made for some hairy moments.
Current campaign I've made a conscious decision to RP conversations with other characters in real time (i.e. rather than saying "I turn to XXXX and ask them..." I just turn and ask them) which helps eke out a bit more character development in every interaction.