I guess my thoughts stray to great groupings of characters in books, TV shows, and movies that I love. Rarely are the characters thrown together simply because they're all there at the right time. And often when that is the motivation, the narrative lacks because of it.
Recently, we decided to run a few sessions where we cut all pretense of constructing an organic, 'natural' narrative and gunned for something simple and direct. It's worked really well - while serving to free up the players to focus on 'getting stuff done' (killing things, rescuing folks, retrieving shiny things) its simplicity has become a boon, providing a framework for the players to build upon creatively as and when they wish.
The setup is as follows:
- The characters are ADVENTURERS
- They are members of the ADVENTURER'S GUILD
- They are given QUESTS by QUEST GIVER (A npc who's name has never been asked and is referred to within the guild as quite literally, 'Quest Giver'.)
- Players agree to have their characters work together because of their shared affiliation with the Adventurer's Guild. They are free to create new characters before each Quest is started.
The players have made a social contract to accept a given quest, with the caveat that each quest is relatively loose in structure, typically with a single goal and a few optional objectives that they can go for if they wish to do so.
Each Quest is built to last around 2 sessions, though each has taken 3 due to roleplaying and a few encounters that escalated in complexity.
My group and I are about to start a new 5e campaign. We were talking about our old 4e game, and the problem we had of the characters being so different in their motivations that there was no real good reason for them to travel together. Why, exactly, is the dwarf paladin still traveling with the gnome sorcerer who sold his soul to a literal devil? (Other than the fact that their players really like their characters, and really like playing together!)
How have you solved this problem in your games?
Some ideas we've had:
- Shared group background. In addition to each characters' background, the group picks a background for the entire party. So for example, your paladin has the background of Noble, but the group picks Urchin. So despite your paladin coming from a noble heritage, they start the game as a gang of street urchins.
- Related characters. We've had this in a couple campaigns already, but being related is a great reason to adventure together. Already for our 5e game, we have three dwarf cousins (a fighter, a cleric, and a... wizard, the black sheep)
- FATE-style backgrounds. When coming up with your character's background, incorporate at least one other character.
Any other ideas?
[*]Related characters. We've had this in a couple campaigns already, but being related is a great reason to adventure together. Already for our 5e game, we have three dwarf cousins (a fighter, a cleric, and a... wizard, the black sheep)

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.