Most Cthulhu adventures, whether Trail or Call, assume that the events the protagonists are meant to be investigating are all basically true. Very rarely adventures like The Westchester House for Call of Cthulhu - seen in the The Secrets of San Francisco and The Asylum and Other Tales - tackle the fraudulent side of hauntings and occult phenomena, but this is very rare. Usually the scenario takes the funny business at face value; there are no frauds, no trickery, nothing that isn't above board. If the initial indication suggests that vampires are the scenario's Hook, they're probably also its Awful Truth. Psychics are genuinely gifted, all Mythos knowledge is by definition Knowledge with a capital K, and everyone's telling the truth no matter how outré that truth may seem.
Bookhounds of London is one of the few settings that takes as part of its subject matter the issue of fraud. The protagonists in that setting can forge books, cheat their customers, and generally commit any kind of deceit in search of hard profit. It also assumes that protagonists can be taken in by frauds; after all, without the possibility of trickery there's no use for abilities like Textual Analysis. However even Bookhounds doesn't spend a lot of time discussing frauds, and abilities like Forgery tend to lead players to believe they will be the ones committing fraud, not that cheats will try to trick them.
That isn't reasonable. If the setting is to resemble the real world in any way, shape or form then the tricksters ought to outnumber the truths by at least 100 to 1. Given that, there ought to be plenty of people trying to take advantage of the players, and any number of fake Mythos secrets floating around. If that doesn't happen then there is no real mystery; just a sequence of events that leads to a climax scene. Without the possibility that the protagonists might be wrong there's little dramatic tension, and the best way to demonstrate that to the players is to offer up some actual frauds for them to deal with.
There's a section in Flim-Flam that talks about psychic surgery, as practiced by tricksters in the Philippines, Brazil and elsewhere. These are people who practice faith healing, claiming to remove tumors and cysts with little pain and often little or no surgical tools. Instead they reach into people's stomachs and remove the offending cancers by hand, with a lot of dramatic bloodletting. Randi points out that this is best achieved with a fat patient, since the flesh can be kneaded and folded in such a way as to hide the slight-of-hand used to produce the chicken guts and other substances the surgeon has on-hand to convince his patient. Naturally they expect payment for this service, and when Randi was writing about them the surgeons refused up-front payment but took their cut in the form of donations and fees for the use of surgery rooms and other services.
Where this comes in for a setting like Bookhounds is this: health frauds have always been popular, and often prey on the most vulnerable - the poor - who lack the resources to get anything better. This would have been particularly true in pre-National Health London, where the only way the poor could get medical services was through charity and Poor Law offerings. This is also a time when abortions were illegal and poorly understood; when surgeons were still feared as patients went under the knife and never came out of the operating room alive, with little explanation as to why. In that kind of environment quackery flourishes, and in a horror setting what better quackery than withcraft with a Mythos element?
So, let's talk about a potential NPC: Cephas Norwood, alias Albert Pinckney, a South London trickster. Cephas puts himself forward as a faith healer from Trinidad who has access to secret knowledge passed down from his witch doctor grandfather. He occasionally hold seances, but this aspect of his business has died down almost two decades after the Great War ended. These days he gets most of his money from cures - and, in a Sordid setting, abortions - sold to the working poor of London. Cephas claims to get most of his knowledge from the guiding spirit of his grandfather that he keeps in a rum bottle, but he also has his Dream Book for reference in extreme or obscure cases.
Abilities: Assess Honesty 6, Art: Music 1, Bargain 4, Biology 1, Credit Rating 2 (4) Conceal 8, Disguise 8, Filch 10, Fleeing 8, History 1, Intimidation 4, Medicine 1, Occult 2, Oral History 3, Pharmacy 2, Reassurance 4, Scuffling 9, Theology 1, Weapons 9
Damage: Fist -2, Knife -1
Cephas uses Disguise to make his skin seem much darker than it is, and Intimidation combined with Occult to bolster his reputation. His actual Credit Rating is 4, but he pretends to have a lower Credit Rating to fool gullible people. Allegedly he lives a humble, ascetic life without need of cash but he burns money - actually a Filch-based scam - in front of his grandfather's bottle to appease his spirit and buy power to heal the sick. He's supposed to have put curses on several people, all of whom suffered terrible accidents of one sort or another. His big trick is to cure people of diseases by "pulling" the sickness out of them, often with dramatic special effects. In Sordid settings abortions are carried out in much the same way, but are accompanied with a "health-restoring purgative" - actually a herbal mix based on large quantities of Tansy - to induce a miscarriage. The mix is highly toxic, and unless the subject makes a Difficulty 4 Health check they take +2 Damage. Though Cephas has never been further West than Soho, he uses slang terms to bolster his alleged Trinidadian heritage, words like:
- duppies: ghosts
- doogla: mixed race person
- eh eh: exclamation of surprise
- pan: steel drum band, a sound which was invented in the 1930s and which Cephas claims proficiency in
- vex, vexed: angry, upset
- wah-jang, wah-bean: slut, loose woman
Someone like Norwood could be encountered in several different ways: as a self-proclaimed expert, a potential mystic adversary - perhaps even trading "curses" with the protagonists - or even as a customer. After all he does have a fair amount of Credit Rating, and he must be spending it on something ...