A Few Words from the Keeper
This is not D&D
The major difference between CoC and D&D is that CoC is firmly grounded in the real world. Yes, fantastical things will sometimes happen in this world, but it is still our world and, therefore, follows certain established rules. It is unlikely that your investigator will be able to lift 1,000 pounds over their head, or be able to jump twenty-five feet in the air. The only three-foot-tall humanoid you are going to meet on 42nd and Broadway is a toddler or a midget.
Human beings are fragile and will be treated accordingly. Gone are the days of 140 hit points and phials of healing potions at the ready. Minor cuts/bruises will heal in days – major injuries in weeks/months. Serious injuries that are not treated immediately will have dire consequences. Blunt, piercing, and slashing weapons all have the ability to kill with one attack. If you wouldn’t engage a group of heavily-armed criminals in real life, it might not be a good idea to do so in CoC.
Your investigators will not gain levels or experience points; they will gain knowledge. If they’re not careful, this knowledge will slowly chip away at their sanity until it leaves them completely unbalanced. Successfully completing a scenario will earn your investigator sanity points, skill points, money (occasionally), and in some cases, mythos tomes/magical items.
The pen is mightier than the sword
The padded cells of Arkham Asylum are occupied by well-meaning individuals who rushed headlong against the mythos without first trying to understand it. Certain scenarios require much more than firepower; thoroughly reading, researching, interviewing, and searching will go a long way towards keeping an investigator alive and sane. That’s not to say that your investigator shouldn’t carry a weapon; sometimes they can get a lot further with a kind word and a gun than just a kind word. However, scenarios can be successfully completed without resorting to violence.
Don’t get too attached
Due to the epic and heroic nature of fantasy-based role-playing games, a player may develop a very personal relationship with their character. So much so, that the untimely death of that character may send shock waves through a campaign, and even cause great distress to the player.
Players should be forewarned of the fragility of the CoC investigator. In any scenario, an investigator can be permanently incapacitated by a physical or mental injury. This shouldn’t be cause for alarm; if the role-playing is strong, and the threat of the mythos remains hidden from a blissfully ignorant world, then the death of an investigator need not be a source of distress. The death of an investigator should reinforce the idea that thorough investigation, sound planning, and solid execution are all keys to helping your investigator live to fight another day.
We live in a society, people!
Law enforcement is alive and well in the 1920s. While DNA testing is several decades away, forensics (newly practiced in the 20s) and good-old-fashioned detective work exists in the Jazz Era. All people (in theory) have rights, and violations of those rights have consequences. Guns can be traced, fingerprints can be dusted, and witnesses can ID a suspect from a police lineup. If an investigator wants to avoid a long, messy trial, certain precautions will need to be taken.
While it might seem that role-playing in the 1920s doesn’t seem like a lot of fun, the inclusion of the mythos, combined with the most user-friendly rules in the known universe (no more confusion when grappling that troll!), makes CoC a worthwhile experience. If you can imagine it and it’s physically possible (and if there is a skill check or checks that supports it), odds are you can do it in CoC.