Okay, so you've put the players through their paces, they've become comfortable with the setting and the mechanics, and they've just become aware of the major plot arc. What next?
Let's take a step back and talk about antagonists again, with Sarah Montgomery and Stanley David Fentiman as examples. Why were they chosen?
Well, in part they were designed to match the setting.. Each setting has its own quirks. An early Gothic locale with crumbling castles out in the lonely countryside is very different from a bustling coastal city in the glory days of Haroun al-Rashid (may he live forever), and both are as different again from an antiseptic space station out on the periphery where the least breach of delicately balanced artificial systems could mean death for everyone on board. Each setting demands a particular kind of antagonist. Sauron only really makes sense in Middle Earth; he'd be out of place in the Wild West. He could be made to fit, but in the doing of it he'd stop being Sauron and become something comletely different. A mad AI determined to eradicate disease by eliminating potential carriers would be great in the space station, but wouldn't make nearly as much sense in Baghdad, and so on.
This is Bookhounds, and that means buying and selling, with the characters as middle men, taking their cut from each transaction. Buyers and sellers are the best antagonists in that situation.
Sarah Montgomery is the buyer. She has wealth, and that often translates to power and influence; she's used to getting what she wants. That means she can put pressure on the protagonists in ways most customers can't; she can cause them legal troubles, financial problems, pay for thugs to smash up their store. Equally she can reward them far beyond their wildest dreams, should she wish to. Both are powerful motivators, but some Keepers may wonder, why make her as rich as all that? Surely she didn't need to be so wealthy to be an effective antagonist?
Yes and no. Yes she could be effective without wealth, so long as she had enough money to be interesting to the characters. However it's more useful to the Keeper if she's more powerful than the characters and their allies put together. Someone living in the hobo shack down by the railway tracks can be beaten up, or have their possessions ransacked, and not be able to retaliate. The players know this, and if they think the antagonist is weak they'll probably go on the offensive. Usually the Keeper doesn't want this to happen. There are times when attacking can be a lot of fun to play through (there's a reason why Dungeons and Dragons is still one of the most popular tabletop systems out there) but in horror games direct assault ought to be the last resort, since there are too many ways it can go wrong. Wealth is a clear indicator of power, and power doesn't get attacked very often, at least not directly. Making Sarah wealthy - which means she'd have all the trappings of wealth, including bodyguards, assistants, a large estate out in the countryside where everyone's her friend or ally as she's the Lady of the Manor - signals to the players that direct action isn't the way forward.
It also means, in a game world focused on buying and selling, that she's an attractive target. After all, when someone can afford to pay three or four times the going rate, and not quibble about the price, the vendor starts paying attention. That's a pretty tasty reward for a day's work. If the characters want it, they need to do what she wants them to do. Someone with less money would be less interesting. Why should a vendor go out of his or her way to satisfy a customer who always demands a discount and never pays list price if they can help it? There are other customers willing to pay for what they want.
The fundamental principle is this: just as the antagonist needs to fit the game world, they also need to be powerful enough to be useful to the overall plot. The mad AI on the space station has no wealth at all. That doesn't matter: it controls all the station's systems, including life support and security. In other words, it has power that is appropriate to the setting, which in turn means it can directly affect the plot. The plot needs to be paramount, in a story-driven game; the plot is what drives the characters forward, what gives them their motivation and their goal. Without it, they're adrift, and the game suffers.
So back to Fentiman. If Sarah's such an effective protagonist, why is he here?
Well, in part he's here to provide Sarah with knowledge she doesn't have and can't get with money alone. He's the one with the occult skill, and she has the resources to make it happen. In theory he didn't have to be there: she could have had the occult skill herself. However that would raise more questions than it answers, chief among them being: how did she get that skill in the first place? It seems out of character for a wealthy widow to somehow have all the knowledge she needs as well as the money. Genuine polymaths do exist, but they're incredibly rare. To put it in modern context: Lord Sugar is a businessman with decades of experience behind him, but that doesn't mean he could design a computer system, or build an office tower, or win the Grand Prix. All those achievements take a particular skill set, and it isn't reasonable to suppose that he would have them all, any more than it's reasonable to suppose Sarah can do everything she wants to do. It also begs other questions, like 'if she had magic powers why didn't she protect her sons during the War' or 'doesn't she realize that what she's trying to do involves dealing with forces she can't control' or even 'so why does she need us Bookhounds, if she's already got the power she needs to make it happen?'
Whereas if Fentiman is involved things start to make more sense. Yes, he has the natural ability, but he's always lacked the funding necessary to get what he wants. He probably knows a great deal, but the real secrets have always been just out of his reach, so he needs the Bookhounds or someone like them to hunt up the grimoires he'll then use to further his schemes. He is her agent, but he has his own plans, and that means he can complicate the players' lives in ways Sarah's never dreamed of. She has lawyers and flunkies: he can summon up the powers of the Mythos. She can interfere in their day-to-day; he can infiltrate their dreams and plague them with nightmares. Again, his power is appropriate to the setting, but in a completely different way. He represents the true threat, in a horror game: the one who can plunge everything into chaos, and will, to get what he wants.
That's it for the moment: one more to follow.