Ist person: I (as in from the perspective of the main character)
3rd person: He/she/it (From a not-involved onlooker)
2nd person, (You), would therefore be from the perspective of an existing character controlling another
For instance, a game where you play as a camera guy in a documentary or record of a hero. The "camera guy", in this instance would exist as a third party, essentially fulfilling the roll of a third-person over-the-shoulder camera. In contrast, this "camera guy" is a character in the game world and, while the hero is controlled by the player, the "camera guy follows him/her around. The camera guy is just as essential as the hero and can be attacked as well, both being able to lose health (though camera guy likely being the more fragile of the two). The camera guy would help and interact with the environment, sometimes backing off to get a better shot (or avoid danger), but it is clear who the main character is. The game essentially boils down to a combination of defending the camera guy and completing objectives.
This probably hasn't been attempted much as it is untested (commercially), it may not be terribly fun to play and it could end up being absolute hell to code (that last one, perhaps not so likely).
The only example of a similar concept I can think of is Rayman: Raving Rabbids (I can't remember which one). You play as one rabbid attempting to direct another with two bike horns so that it hits as many obstacles as possible within the time limit. The horns control the movements of rabbid 2 while the camera is from the perspective of rabbid 1, who backs up as rabbid 2 moves foreward. Ubisoft could afford to do this as, if the minigame was a total failure, there were plenty of others to pick from, though I can't really see this idea going on to make a complete successful game.
Of course, I'll eat my words if "The life and times of Typical Action Hero #562" is the hottest selling game of 2014.