*: Recommended House Rule: The rules say MOS is once per session, but I’ve found that once per MISSION works much better, especially with larger groups. Once per Op means the players have to be a little careful not to just blow their MOS on something trivial, and in turn that frees up the GM to really make those MOS activations knock everyones socks off. The best reason to go with once-per-Operation instead of -per-session is that this discourages players from dragging things out. Someone metagaming the refresh system of a Gumshoe game like NBA can actually wreck your campaign. You never want a situation where the best play is to turtle up and do nothing until the end of the night because next week the PCs will all be back at full strength if they just stall out the clock. For my own campaigns I’ve moved all the end-of-session refreshes and processes to happen at end-of-mission instead, and I find it’s a strong improvement. Ops rarely last more than a second session back-to-back anyway, because all those MOS activations and Cherry benefits generally allow the PCs to shoot for the moon and stick the landing on any plan in just a session or two. About the only situation where I would consider moving things back to a per-session basis would be if I was running a campaign with just 1 or 2 players.
I’ve seen three main philosophies or schools of thought for player MOS. Having seen them all in play, I think all are equally valid, though it took me a little while to get to that. Here’s three ways to pick an MOS:
1) Use it to strengthen the skill you plan to spend the most frequently. This is the obvious play for a super-shooty assault specialist. Putting your MOS in a skill you plan to lean on heavily provides you the freedom (and safety) of being able to blow all the points you want on every roll, knowing that even if you bottom out your best ability in a protracted conflict you still have one helluva trick up your sleeve.
Though I mentioned shooting, this approach actually works super well for non-combat skill MOS assignments. The notion of “master of disguise” is a very fitting one for the genre, but it can be very point-intensive in this game. You may feel that Disguise alone isn’t good enough, and before long your core concept is eating up your General Ability budget with the smorgasbord of Cover, Network, Infiltration, and Surveillance. Covering all the aspects of “master of disguise” can leave you feeling stretched thin, and those points all feel a little wasted when the current Op is some unsubtle smash-and-grab. Putting your MOS into one of those Abilities is a great way to ease that pain. You can then afford to specialize in just the Ability levels needed to score the most appealing Cherries, and know that a timely MOS will cover any oversights or point deficits.
Aligning your MOS with your best (or one of your best) skills feels very fitting, and makes good sense in- and out- of-character. There is no rule forcing you to pick an MOS in your best skill though, or even in a skill you've got any points in at all. Let's look at some other philosophies and options for MOS selection...
2) Use it to shore up a critical weakness. Here you’ll take a MOS in some action skill that you are otherwise incapable of using. The first time a player in one of my campaigns picked an MOS they didn’t have more than a point or two in, I was very apprehensive. On some level, it felt almost like an abuse of the system. But now I’ve seen it in action, and I’m a full convert.
One of the PCs in my current campaign took a Weapons MOS and basically no other combat skills. Once per mission* she can get the drop on someone and beat them into submission with a frying pan, but if faced with a protracted battle she’s more properly motivated to surrender and gather intel as a prisoner. It works out pretty well. She gets fun spotlight moments in the occasional fight despite being otherwise a non-combatant. That is all kinds of cool. The MOS mechanic allowed her the freedom to play a quirky civilian in a setting where that might otherwise be a lethally bad idea.
3) Use it to hand-wave past your least-favorite part of the genre. The entire point of Gumshoe is to cut out the tedious die-rolling and related frustrations that can wreck a mystery scenario… so there’s no reason you can’t apply that principle to any skill or type of scene that just doesn’t excite you.
For the sake of the argument, let’s say you just hate car chases. Maybe it’s the chase mechanics never live up to your imagination and expectations. Or maybe you’re bus-bound in the real world and couldn’t give a damn about cars. Whatever the reason, you’ve decided that you can’t stand car chases and don’t want to play that minigame. It may be counter-intuitive, but in that situation an MOS in Driving would be a great investment. It would allow you to short-circuit any chase or tailing sequence you want by just invoking your MOS to escape/catch-up/ram the opposition. If there’s someone else at the table that absolutely loves car chases, you can expect that there will be the occasional twice-in-one session chase scene extravaganza, but at least you only have to suffer through the second half of the double-play.
(EDIT/afterthought: Depending on just how much your fellow players enjoy a car chase, you may find it fairest to instead suffer through a few minutes of it in the early part of a session, and then invoke your MOS if the scene really starts to drag or if a second car chase crops up in the same session. Your mileage may vary.)
Driving was merely the low-hanging fruit there, and the same principle can be applied to nearly any type of scene you don’t like if you just target the skill most likely to shortcut it. Hate shopping and planning scenes? A Preparedness MOS will get you the right tool for the job with no advance notice. Can’t talk your way out of a wet paper bag? An Infiltration MOS will get you past the security check point and on to the fun parts behind enemy lines. Completely bewildered by technology, or bored to tears by hacking scenes? Digital Intrusion MOS cuts those down to a quick montage and a bare minimum of jargon.
The MOS mechanic is one of great innovations of Night's Black Agents, and it does an amazing job of empowering the player to not just customize their character, but also tailor their gaming experience just the way they want it. That's a win for everyone.