In my experience, Callers work better in face-to-face games, as a way to focus the group (that can rapidly get sidetracked into conversations and other distractions). PbP doesn't suffer from that.
It DOES suffer from waiting on individuals. Adding an individual that the DM is waiting for isn't a good thing, IMO.
What's worked in the PbP games that I've been a part of is a guaranteed 'bump day.' A day in the week where the DM moves the plot along based on what's been discussed. Now, that's not to say the DM can't move the plot along faster if the players have been posting lots.
It's a friendly way of saying 'we won't wait for you,' but also 'it's okay if you miss a week here and there.' Does that make sense?
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PbP Advice for our new-to-PbP DM:
Momentum is the most key element of keeping PbP going. Slow or fast, a steady rate is the surest way to keep a game kicking (that's why the 'bump day' method works well.
Which leads me to the The Player's Corrolary.
The Bump Day Corrolary: It is ALSO the players' responsibility to maintain the momentum of a PbP game. If a good span of time (a week is a good baseline) but you're not posting because you're waiting on other players or the DM, it's time to post again.
Sometimes players need to wait on the results of a roll or something like that, but that doesn't mean you can't post. Post an inner monologue. If you're character's not introspective (or you as a player don't like posting in 3rd person omniscient), post something more about the events that are happening. If the group isn't in combat and other players are lagging, it is important to keep the group moving, make a decision. (Sparky note: I don't want to hold up the game, so if I miss posting and you guys do stuff that my PC wouldn't have done or supported, I'll roll with it, because there's a meta-contract between all of us that we want to do this together. That means, if I'm not posting, I don't get a say.)
A post that lets the players and especially the DM know that people are still checking the game threads and invested. OOC posts don't count. IC posts do.
Ahem.
That was long winded.
I'm sure we'll all be just fine.
Working up a Leader.
Preferences? Cleric? Warlord?