I say "NPCish" because they won't be filling what I usually look at as the NPC role. My traditional view of the NPC role is "characters who may provide information, challenges, conflicts, and/or companionship to the PCs, but who will not usually offer ideas or solutions to problems, or take the spotlight, or be better than the PCs at their core competencies, or otherwise upstage the PCs." The list of "don'ts" is mostly because NPCs have too many unfair advantages over PCs: NPCs are played by the GM or by someone briefed by the GM and so have OOC insights the PCs aren't privy to, and NPCs aren't necessarily limited by the same character writeup system as the PCs. It's very easy to have an NPC upstage the PCs if the GM wants to, and most players find it annoying if it happens.
The thing about asking someone else to play traditional NPCs for me is that it often doesn't make GMing any easier. I have to brief the other person on all the background they'd need for the NPC, which may mean writing out notes that formerly existed only in my head, and I have to field questions on whatever subjects I've left out. Moreover, the actual play of the NPC is often the most fun part for me, so it's like trading doing more of the tedious work of pre-game prep in order to get out of the fun part of playing the game. It often improves the game as a whole, but isn't really a win-win for me.
So what I want to do instead is a different non-player role. These characters wouldn't be played by me, and they wouldn't get any special behind-the-scenes insights or information. So they wouldn't be able to do the traditional "provide information to the PCs" role. I haven't decided yet if I'd let them conflict with the PCs, but probably not. But what they could do, in addition to being companions to the PCs, is provide the PCs with ideas, solutions, and a useful sounding board for IC discussion.
The main thing that makes them not PCs is that I won't really be paying attention to them. They will exist primarily to interact with the PCs, not with the GM. If these characters wander off on their own, the camera's not going to follow them and I won't be running sub-plots for them. The characters will generally be expected to go along with the wishes of one or more PCs; if they really want to try an idea on their own, it will almost certainly fail ignominiously and off-camera. >:) There may be exceptions: if it happens that I have free time available and ideas, I might have actual NPCs interact with them or otherwise treat them a bit more like PCs. And if they're in a scene with PCs and regular NPCs, the NPCs aren't going to ignore whatever questions they ask. But in general I won't make a special effort to engage them.
This is pretty much the "Supporting Cast" role that I mentioned in my first poll. "Stars/Supporting Cast" was pretty unpopular in that poll, but I am guessing this is because either (a) people didn't think it would help resolve problems and/or (b) most people were not personally interested in being relegated to a supporting role. Hopefully, no one actually objects to other people volunteering to play supporting characters, because I've got at least one friend (telnar) who's specifically interested in that. So I'm going to include this as an option for his benefit if no one else's. The reason Telnar prefers an SC is that his schedule won't necessarily allow him to post daily, and with an SC he doesn't need to: if he's around he can contribute, and if he's not then the group can assume he's being quiet and going along with whatever the PCs want. (Or the GM can take over for him, if need be). In Honored, octantis (playing Inkwell) and playfuleye (playing Mongrel) more-or-less had SC parts, although I hadn't formalized it like this and Inkwell actually did have his very own sideplot, albeit one that moved very slowly.
In other gaming news, I haven't decided which of the game options I'm going to use. I'm leaning towards "Dealing with the Unknown", mostly because it will require less worldbuilding and pre-game preparation than the other two ideas. >:)