For instance, the characters need motives -- BUT -- not too much motive. Particularly for the less important characters.
If a character has a wonderful backstory for adventures, the other characters may need to share it, or else not have such a wonderful backstory. If one character was living in a town when an elderly paladin died of old age, and begged that the character take up his sword Rest-Bringer -- which will give her the duty to lay to rest good ghosts by resolving the bond that keeps them bound to this world -- well, it's wonderfully productive of adventures, but either the entire party had to have been there and received the charge (and perhaps magical objects -- perhaps the paladin used to have comrades in his quests, and carries the objects that helped the dead comrades in their various classes), or else, they have to have motives that allow them to subordinate themselves to the new paladin's quest.
Whether their king or church told them to assist her in her noble course, or being the sole survivors of a massacred family has them level-grinding to take down the enemy responsible, or even thinking it a good way to fame and fortune -- there are a lot of subordinating motives. Still, there does arise the question of why they don't find an easier way to do their individual objectives. So they need motives for their motives.
To be sure, if the novelist decides to give them all a common motive -- all took the commission from the dying paladin, or all survived a massacre and are trying to avenge the dead or escape the assassins (or both), or all won magical objects in a battle with a monster thus giving the king they serve reason to believe they can do great deeds for him -- he doesn't have to worry about introducing new random characters because the luck of the dice turned against one character.