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When I was a wide-eyed assistant, I used to hear editors mention submissions that scouts were "buzzing about" or were "high on." And I would think to myself: what the hell is a scout? They seemed to be everywhere and nowhere all at once, like some kind of magical gossip elves. And who, exactly, did they work for? A scout would call my boss after our editorial meetings and within moments, I'd be covertly messengering over some proposal or manuscript to said scout's offices, making extra certain that it could not be traced back to my boss. (Who said publishing isn't exciting?!)
Now that I'm an old publishing troll, I've gotten a (modestly better) handle on the whole scout fandango:

Scouts are the spies of the publishing world. They're like James Bond, except without the sex, drugs, guns and generally any of the stuff that makes James Bond cool. Instead of a license to kill, they have a license to KNOW EVERYTHING and EVERYONE. The good ones are charming and sly. They are paid by foreign publishers or movie production companies to get the scoop on projects that are out on submission and determine if there's any movie or foreign sales potential. They do this by making nice-nice with editors (and some agents) around town, cozying up to them so that the editors feel safe
enough to share submissions (that agents have not already leaked to the scouts.) They then feed these submissions to their clients. (And, sometimes, they even feed editors. Lunch. And alcohol. Which is appreciated, since we editors usually pick up the bills for publishing-related outings.)

Why do editors love scouts? Because they're the rumor-mongering whores of the publishing world. In a totally good way! You want to know whether any other editors are really interested in a mediocre proposal you suspect an agent is bluffing about? A good scout will know. Curious exactly how ridiculously that rival house overpaid for the novel you really wanted (or didn't want at all)? Ditto. Scouts can also let you know which projects are worth looking at, which is especially useful if an agent has neglected to send said project to you (that asshole). Bottom line: They have the best goss, period. No one gets intel like a scout.

Why do some agents HATE scouts? Scout chatter can kill a project dead. See, agenting often involves bluffing. This can mean anything from exaggerating about rival publishers' interest, or making up offers (only the lowest of the low do this — it's not only highly unethical but also just totally stupid and icky and deranged), or, most common, blowing sunshine about a proposal that's undeniably craptacular. By virtue of their job description, scouts can pull up the agent's proverbial curtain faster than you can say twatwaffle.

So be nice. Or face being kept in the dark about the gossipy-goodness that is the cornerstone of our industry.

Unsolicited is an anonymous editor who still works in publishing, aka NOT EMILY. But don't worry; Emily dutifully forwards your hatemail.