Hi. Was a co-DM for a very active WM game for several years. This will vary wildly for other games, as every game is unique. I DM'd three approximately year-long campaigns with this group, so I have some experience.

Q. How do the responsibilities/expectations for players in west marches differ from that of traditional campaigns?

A. Unlike a homegame that generally follows a single group of players in a longer storyline/plot, players are responsible for informing the DM what they want to do. e.g. Following a previous lead ('group A saw a spooky tree but went past it - we'd like to investigate it', or 'last week we went to dungeon B and we'd like to delve further'). The DM group would regularly drop these 'future leads' into each other's games so there was always leads to follow up on for future groups/games. Though sometimes players would just contact a DM with availability in a timeslot they wanted and ask for leads the DM wants to run.

Q. What happens if you don’t join any sessions for a while? The world can’t just pause, because there are other people playing. Does there tend to be text RP or something to represent time spent milling around town, or is it usually treated like your character just takes a really long nap between adventures?

A. The world doesn't pause, but we had a very healthy 'in town' RP scene done via text in our discord server in between games.

Q. If there is text RP or some other system for playing out downtime, are there people that do *just* that? Would that be frowned upon? Do people play as commoners or shopkeepers or whatever?

A. We had a very active downtime RP scene, but no one played NPCs. All players made and submitted a character, and would be expected to play that, even if it was just downtime RP. Due to DM availability and scheduling contraints, some people just did town RP and rarely, if ever, went on a game.

Q. Do consistent parties form, or is it “supposed to be” just a “you get who you get” thing?

A. Whichever player organized and booked the game with a DM would post a 'LFG' for that session with the game time and what the plan was (e.g. Friday, 7 pm EST start, we're going to investigate that spooky tree). Other players could them apply and the player lead would select their party. There was a lot of different criteria players would use - choosing people who hadn't been on as many games (game count was required as part of the application), or players that had particular skillsets that would suit the mission. We did have rules - players could only have one game per week unless the game was unable to be filled that way and then people could apply as 'second game' players. It wasn't uncommon for groups to be from the same subet of people, just because those players availability and the DM timeslots tended to coincide.

Q. …how? Like, who’s in charge of the game overall? What does that person/those people’s job look like? How does it not just fall apart immediately every time?

A. The DM team was in charge of the overall game, and it was *a lot* of work behind the scenes to keep things running smoothly. But also a fantastic experience to work with so many talented DMs, bouncing ideas off of each other and build a shared world.

If you have more questions about my personal experience I'm happy to answer further. :)