Let's use some specifics from my recent PC planning sessions (both in-person and via email) to illustrate some of the points from my prior post. Some of the names have been changed to protect the innocent.
Player 1 is a roleplayer and enjoys both story elements and gamist elements and is a pleasure to have in the group. She dove right in to my background and setting information and came up with a concept (scoutlike halfling rogue) that was tightly connected to the gameworld. She developed a few NPCs (family members) and gave me some additional ideas for the campaign, writing out a backstory (and names!) for herself and immediate family members. In this way, she became a contributor to the campaign itself. I will have no issues connecting her character to the game or story elements and she provides her own motivation. I just have to make sure I can incorporate her creativity and keep up with her work product!
Player 2 is a blended player with both roleplaying and powergamer tendencies but doesn't really have the time to create the extensive backstories of Player 1. He told me what he wanted overall (a thuggish half-orc barbarian with a revenge backstory) and was eager for me to help fill in his details. Via mutliple emails, we worked through his origin, came up with a name, and explained how his character has narrowly avoided imprisonment and being ostracized in the predominately human and halfling campaign area. He also asked for a specific supernatural ability -- a sort of sixth sense -- we negotiated this and I have a secret "disadvantage" that I'll be springing on him in mid-campaign. While I had to do a little more work with him on character creation, he will be relatively low-maintenance once the campaign begins so long as I can keep the revenge element present in a significant portion of the adventures. We may have to shift his focus if and when he gets his revenge.
Player 3 is one of the DMs in the group and is a flamboyant roleplaying, team-focused, and storyteller type. Typically, he waited for everyone else to declare their concepts and chose the "leftover" role for balance purposes and (predicably?) ended up as the cleric. While I had something in mind for him, he very much wanted to go his own way and created a character with a dark yet whimsical secret. He created his character but left the details of his insertion into the campaign up to me. In this way he can be both a team player (as a cleric) while indulging in his desire for playing over-the-top, humorous characters. As with Player 1, Player 3 will not require a lot of maintenance, providing his own motivation and enjoyment.
Player 4 is a returning player who last played in 3.5 edition games. He's a powergamer that enjoys playing chaotic, id-driven characters as a way to blow off steam and have fun. An analytical type, he is also one of those hyperintelligent players that likes to see if he can break the game. Fortunately, he is good-natured and recognizes that creativity is not one of his strong suits. I suggested a concept that fit his predilection (a Mad Martigan swordsman type with a background as a minor knight -- a character that I originally had envisioned as an NPC) and he was off to the races, diving into the character creation rules to maximize his character's potency. He actually appreciated me supplying him with a concept, background, and name. So long as I provide him with an opportunity to engage in his wild side and give him a chance to be powerful, he will be a fun and non-destructive element in the game.
Player 5 is a sardonic, introverted player with interest in both gamist and story-driven elements, though she is not a huge roleplayer. She often plays a particular type of character (usually an elven druid or other support-oriented divine caster) and has decided to "branch out" a bit by playing a human bounty hunter that found religion/philosophy and became a monk. As with player 2, I will likely be doing back-and-forth emails with her to get the story elements to her liking so she can enter the game fully-formed. My challenge will be to provide her with engaging story elements and combats that allow her to be rewarded for her non-standard character choice.
Player 6, while a good low-key "blended" player, is relatively disengaged by the character creation and campaign launch process. He hasn't replied to my many emails and has committed only to playing a Wizard. No race, no name, no background ... utterly generic. His wife (player 5) has warned me that he'll procrastinate to the end. So I can either keep paddling upstream to get some more information out of him, simply assign him a character (as I did with player 4) which he will likely not appreciate ... or I can go with the flow and challenge my own need to develop everything. Maybe I can let go. Maybe I can just wing it and let Player 6 be the mystery man in the group. Since I am going for a "sandbox" type of game allowing the PCs to do as they will unbound by a central story, maybe having a "sandbox" PC is okay too ... the PC that is in the process of discovering himself or possibly even revealing himself to himself via a curse or mental illness.
So there it is -- this process of character generation and campaign launching, played out with real players with differing motives and personalities. By cooperatively working with the players to give them what they want -- but within your previously-established framework -- you maximize the likelihood that they will take ownership of their fledgling characters and the campaign. This in turn will generate many story ideas and character interactions that you would not have imagined on your own, turning the campaign into a collaborative process.