How does it differ from Trail of Cthulhu and the other horror Gumshoe games?
So first of all I love both Gumshoe and Brindlewood Bay for mystery games. So this is a game that is trying to fill a different niche from either of these.
Gumshoe is great for the kind of setting it's trying to work with, which is one where finding the clues is part of the trail to get to the "action" of the game. In a D&D analogy inn Gumshoe I see the clues as the corridors that are being explored but the "action" of the game is still in the confrontations that you face along the way. Gathering the clues is as assumed in the game as traversing corridors is in D&D (including the fact that you can take actions and spend points to get bonus clues that you don't really need to complete the game but make you feel cooler or give you an extra reward - like detecting traps and secret doors in the D&D analogy). This works great for giving you the feel of a police procedural, but IME makes for a boring game if you don't get that action scene somewhere in the scenario.
Brindlewood is great for creating a collaborative mystery, but it doesn't scratch the itch of the player who wants to have a challenge with a solution presented to them and then they work to solve it like a puzzle.
So it started as a thought experiment about what a game for a Poirot or Murder She Wrote or Scooby Doo or Columbo style game would look like if the process of finding clues were as central to the game as combat is in D&D and the "action" of the game doesn't involve actual "action". There's no combat or even having the suspect run off or pull a gun at the end assumed. Just like in the books and TV shows where there's that kind of mystery - the bulk of the book is about finding clues and putting the pieces together, not athletics and combat. So a game that comes from an alternate world where Gygax and Arneson didn't invent D&D but some guys who were really into Agatha Christie did instead.
As I worked on it it turns out the game's dice action mostly involves back-and-forth interaction with suspects and witnesses and the mechanics are about getting around their defenses or improving their attitude towards you to get clues out of them (including red herring clues from the actual guilty parties to try to frame other suspects). There are also physical clues, but it turns out that the mechanics for searching for clues in an in depth way cannot really be made "dramatic" enough for players to care (or if they can I'm not the person to do it). Much like in a mystery novel or Gumshoe, finding clues when you decide to look for them so you can just move on to the action or trying to figure out the mystery is more interesting than trying to come up with some kind of dice game to find clues.
The initial playtests were really positively received by my player who is really into that kind of idea, but it's also a lot of work to develop scenarios for the game because of the number of NPCs I need to create and the clues I need to come up with. To a large degree I need to create a "Monster Manual" of generic types of NPCs to be able to throw into a scenario and I don't have that. So I need to do that, keep tweaking the rules more, and see if it works with a bigger group, or if I just happen to have one player who is a freak in synch with me and nobody else would be interested