In any Keeper's life a little rain must fall. There will come a time when you have to introduce new players to a system unfamiliar to them. These would-be gamers may never have played tabletop before, or perhaps they have, but in a completely different system. How do you, as Keeper, ensure that these gamers have a good time, while also learning the basics of the system?
First, determine what is a core mechanic. Then ensure that this core mechanic is highlighted in the introductory game. You need to do this because, in future sessions, you're going to be spending a lot of time with that core mechanic, and you need to make sure that the players have a solid grasp of it.
Not only is this important for the obvious reasons, it's also important because you'll be introducing a lot of non-core mechanics later, and if you're going to do that successfully the players need to have a decent grasp of the basics. That way you don't spend half the session explaining things they ought already to have grasped.
So what is a core mechanic? In GUMSHOE, for instance, is clue finding a core mechanic?
I'd argue that it isn't, at least not always. Consider: in a typical GUMSHOE game, it ought to be possible to get through the entire session without picking up a single non-core clue. Of course it isn't as much fun playing that way, but if you can play without a mechanic and still successfully conclude the scenario, then I suggest that the mechanic can't be considered core.
But if clue finding isn't core, then what is? Well, that's going to depend on the system.
Take Trail of Cthulhu. There's one mechanic that the players will always use in every scenario. It's not Drives, or clue finding, though both of those things will come in handy. It's the Stability and Sanity pools. No matter what, the players will be spending from those pools in every single game. More importantly, there is a distinct difference between Stability and Sanity that goes right to the heart of the system. You can go into negative Stability, even end up with almost nothing, and by the beginning of the next scenario be just as hale and hearty as ever, psychologically speaking. But if you lose Sanity, it's not coming back.
Moreover Sanity is also linked to Pillars of Sanity, which can crumble under pressure. There are several techniques like Denial, Fainting and so on which the player can use to reduce Sanity loss. There are pools like Cthulhu Mythos that impact on Sanity. With Stability, becoming Blasted can result in mental illness, and so on. In sum, each of these pools, and their loss, has profound impact on how the game is played, on its tone, and on how the characters will progress.
The players need to understand these things. Therefore the Keeper needs to emphasize both these pools in the first adventure, and so has to ensure there are at least one or two Stability-shaking events. The Keeper also needs to ensure that the Pillars of Sanity come into it at some point, thus underlining their importance. So if a player's Pillar is the sanctity of the Catholic Church, for instance, then some kind of blasphemy against the Church in the first session is probably a good idea, since it invests that player. Also, with that player's reaction, the other players at the table begin to understand the importance of their own Pillars, and react accordingly.
None of this is to say that clue-finding isn't important to Trail. However the Keeper needs to ensure that the really important stuff gets dealt with first, and in this system Stability and Sanity are the really important mechanics. So you put them front and center, leaving the intricacies of other important mechanics for a later date.
Passing from Trail to Bookhounds of London, what's the core mechanic here? Is it Stability and Sanity again? Well, no. Both those pools are important, but neither of them are as important as the Auction and bookstore mechanics. Again, the thing to bear in mind here is how often a mechanic is used, and how it impacts on the tone of the campaign. In a Bookhounds game players will be going to auctions very regularly, perhaps even once per session. Their characters will always be concerned with what's going on at the shop, whether profits are up or down, or whether a patron's about to abandon them for another shop.
So you as Keeper need to ensure that, in the very first session, there's an auction. That the shop's future is threatened in some way by a financial calamity, which only the characters' actions can avert. Even better, that there's some kind of prize to shoot for, a rare and valuable text that they can slaver over and do their best to obtain.
So what about something completely different, like Night's Black Agents? The system is still GUMSHOE, and there's a Stability pool, though not Sanity. What's the core mechanic? Again, consider what your players will spend most of their time doing. This thing, whatever it is, must be the core mechanic. It will be the thing that creates the tone of the campaign.
It's not Stability. In fact, in all the Dracula Dossier sessions I've played so far, I haven't even asked for a Stability check. That's because when Stability-draining things happen, it's often when something else equally exiting is going on, and in the heat of the moment I let Stability slide in favor of something that keeps the badass action moment hot and messy.
In this case, the mechanics that have the strongest claim on being core are Heat and Thrilling Contests. Originally a chase scene mechanic, the Double Tap book expands Thrilling Contests to most of the General abilities, creating all kinds of possibilities. Thrilling chases on foot or in a car, thrilling surveillance contests, thrilling digital intrusion, thrilling infiltration, and so on. This is the kind of thing your agents will spend most of their time doing.
The Heat and Lead mechanics are fundamentally similar to Thrilling contests, which helps, but there's enough difference between the two that you really want to emphasize how important Heat is. So there needs to be that moment when the local authorities get involved, and the Keeper can point to the Heat the characters gained so far as the reason why.
Personally, I use red poker chips for Heat, putting them out in the middle of the table. Whenever Heat is gained, I add to the stack. That keeps everyone invested.
Two other mechanics that probably need a bit of special time are the Cover and Network pools. Again, these are pools that the characters will be referring to on a regular basis. More importantly these are also pools which don't refresh, making them very distinct from all the other General pools. Players can sometime overspend on General ability tests to ensure victory, but if they overspend here, they could be setting themselves up for disappointment later. They need to understand these mechanics, so you as Director need to ensure there's a moment in the opening scenario when these mechanics become important.
This is particularly so for players who are familiar with GUMSHOE, but not Night's Black Agents. Look at it from their point of view: every other GUMSHOE game they have ever played has taught them that General pools refresh. Some not as easily as others, but they do refresh. Sanity in Trail is the only pool that doesn't, and in that instance there is never a chance to rebuild the pool. But there is a chance to rebuild Cover and Network; it takes time and is expensive, but it can be done. A player who has become used to the typical GUMSHOE game is bound to be a little confused.
A player who isn't confused is one who's having fun. And fun is the whole point of all of this.
So, in sum: understand the core mechanic, and emphasize it in the first session. The core mechanic is the one you'll be using all the time, that sets the tone, and has a profound impact on how the game is played. Once the players understand that mechanic, everything else will follow.
That's enough for now, I think. Good luck gaming!