Merkuri
Explorer
In our group we had four dungeon masters who switched off running games; all of us had player characters in the party because we rotated the chair behind the screen. My character was not an npc - it was my player character.
We're doing the same thing in my group at the moment, except when we DM, our PC goes off and does something else.
Even though technically it's the same character, I can't imagine playing my PC the same way in a game I'm DMing as I do in a game where someone else is DMing. I think about the character differently. She ceases to be my avatar, and just becomes another set piece. She may still have the same voice, the same powers, the same sense of humor, but she acts differently in a subtle way. She would get a lot quieter while the party is deciding what to do, for example. She wouldn't lead the way into a dungeon, except maybe at the direction of another character.
And that wouldn't feel right to me, which is why, when we were setting up this campaign, I voted to have the DM's PC fade out during the adventures they run. I'm not saying I couldn't play my PC while I DM'd - just that I felt it it would cheapen my relationship with that character while I was DMing.
My point is that a good DM will play their PC differently in such a situation to the point where the character is not really a PC anymore. It becomes an NPC that's a member of the PCs' group. It's the same character that was there last adventure as a PC, but it's not a PC this time.
Again, this might just sound like semantics, but I find that having different labels can help (especially for new DMs). If you treat your PC like an NPC when you DM then you are probably fine, but if you treat it like your PC, along with all the emotional connection that tends to go with it, you have the potential for problems.