Note: For the purpose of this topic, NPC means Non-Protagonist Character, rather than Non-Player Character. So when I say NPC here, I mean a character other than the main protagonists of the campaign normally played by the players.
For the last couple of sessions of my Firefly campaign I've been giving one of my players an NPC I'd written up for the week's scenario to play as in place of his regular character (due to his own character being presumed dead and wanting to leave his fate undetermined while I dealt with another PC's spotlight episode, which ran on for a session more than planned).
I gave him two different NPCs to handle in each session and a quick brief on their purpose within the sessions, which were largely to be a foil to the spotlight character and lampshade her issues. The result was a more focused character portrayal than I think I might have managed myself. The player playing the NPCs had fun playing them, the spotlight character's player enjoyed clashing with the other player's NPCs, and I enjoyed being able to watch their interactions while having some extra freedom to deal with other parts of the scenario.
This is by no means the first time I've done something like this, having had one or two playable NPCs on hand for most sessions of my Unisystem Stargate campaign. It's also a necessity in GMless games such as Fiasco, where there isn't actually a GM to control the NPCs, so players whose characters aren't active in a scene take on the role of other characters who are present.
So what are the benefits of letting players control NPCs, and what are the potential pitfalls?
The first two I've alluded to already, but I'll go over them again just to lay everything out in an ordered fashion.
First of all, it's really handy if you need to introduce a new player to the group but don't have enough time to go through character creation with them before starting the night's session. Or maybe they're not sure about joining the campaign yet, so they just want to play one session to see what it's like. For these cases, it's really handy to have a couple of NPCs statted up before the session that can be used by players, with notes on how they can be introduced and integrated into the party. That way, if a new player unexpectedly joins you, you have a way to get them involved right away.
Second, for your regular players, it can be a handy way of keeping them involved in play when their own character isn't in a particular scene or even the whole session. Not every character can be in every scene all the time, so rather than having them sit on the sidelines and watch a scene unfold (which some players are happier to do than others) you can give them a more active role in that particular scene to keep them involved. Or it might be that their own character in the campaign died last session and they haven't got a new one ready yet.
Third, as a GM you have a bunch of different stuff to juggle during a session, and allowing a player to take control of an NPC is one way you can delegate some of that responsibility and free yourself up to concentrate on other aspects of the game. For example, if you have an NPC who's allied with the PCs then it makes combat sequences simpler for you if you're not having to roleplay and roll for them as well as the bad guys.
First, as GM you can predict the actions and behaviour of your own NPCs, but you can't predict the actions of player characters, and that's as true for player controlled NPCs as it is for your regular protagonists. If you're delegating NPC control to your players, you need to make sure you've a) given them enough information about that character to roleplay them in a way that you're comfortable with, and b) that you trust that player enough to handle the role properly. This is less of a concern with one-shot guest-star characters than it might be with recurring NPCs who need to be played with some degree of consistency.
Following on from there, having a player run a recurring NPC presents another problem, as you risk revealing details about that character that have yet to come to light. This need not necessarily be the case: maybe you just give your player a stat block, some personality prompts and a motivation for the scene/scenario. In games like Fate though, where important details might be part of the character sheet in the form of aspects, more care might be necessary. If you trust the player not to meta-game and use that knowledge later on, then it's all good. If you don't, you might have to redact the details you want to keep a secret from the character sheet before handing it to the player.
There are some good reasons to let your players take control of NPCs for a bit, but there are some drawbacks too. Most are easily mitigated though by giving the players the information they need (stats, personality prompts, short-term goal, etc) and omitting anything you're uncomfortable with giving them for recurring characters (such as secret backstories, long-term goals, special abilities, etc). At the end of the day, it's also a matter of how much you trust your players to take this part of your responsibility from your shoulders, which depends on how well you know your group and how they play.
I'd be interested to hear other GMs' thoughts on this and maybe their own experiences of letting players play NPCs in their games. For now though, see you later!