In the past we've had recurring NPCs in the game, and players have taken over running them if their main character was out of action for a time. (Waiting to come back, etc.)
One problem with having someone else in the driver's seat, as it were, is that they don't always know how to drive.
For example, we had a Whisper Gnome Rogue (1 level of Wizard) who called himself Ignominious T Padfoot. (Iggy, for short). Obviously not a real name, but the write up on the Whisper Gnome says that, when outside their own community they often give a false name, a sort of recurring alias. It was also pretty clear that Iggy didn't really know what his chose name meant when he picked it.
Iggy, as written, doesn't consider himself an adventurer. He's a professional performer, using acrobatics and tumbling, and travels with various shows. He is proud to say he doesn't own a backpack or a bedroll, preferring to sleep in his own bed in his small performer's wagon.
Yet, somehow he often finds himself being asked by adventurers to do clever things for them, things that look perilously close to "Adventure".
He has one level of Wizard, with a Conjuration specialty. There is an ACF in the PHB II that allows a Conjuration specialist to trade in his familiar for the ability to make a small number of 10 foot teleports a day, as Immediate actions. Really neat trick for a circus performer who is in no way a rogue or thief of any kind.
He speaks with a heavy Londinium accent, almost cockney, and is always asking the way to Portabello Road. In short, he has a distinct style, which I think active NPCs should have.
Now, enter a Player who is going to take over. Will he have Iggy charge into battle, as adventurers would, or will he cast a small illusion of a barrel or rock, and hide inside it, as he normally does? (A Whisper Gnome has racial and size bonuses to Hide that make him all but invisible. His bonus was in the 20s when he was 3rd level.)
He's seen far more close fights that he was ever intended to be involved in.
And yet,somehow, he's still somehow a level or so behind the party.