Coincidentally with the release of the latest remake of INVASION OF THE BODY SNATCHERS, about aliens who assume the identities of human victims, I've just read two novels on a similar theme: Hostile nonhuman creatures lurking among us who look (to the uninitiated) exactly like normal people. Both of these books, Melissa Marr's WICKED LOVELY and Karen Marie Moning's DARKFEVER, feature protagonists with the power to see faery creatures in their true form, to which ordinary people are oblivious. Moning's Fae are all dangerous, none of them beneficent toward humanity; the best that can be said is that some of them don't kill us. While some of Marr's Fae have a benign attitude toward mortals, many of them are cruel and most, at best, capricious. Both of these heroines' lives depend on not letting the faery folk realize their illusions have been penetrated. I'm also reminded of A. E. Van Vogt's WAR AGAINST THE RULL. The alien Rull, hidden behind their human disguises, can be recognized by touch, so refusal to shake hands is highly suspicious. And wasn't there a short-lived TV series in which the extraterrestrial invaders looked like the rest of us except for a small anomaly in their fingers?
Suppose you were one of those people who know hostile beings walk disguised among us? If you tried to warn the rest of the world about the danger, you'd be classified as delusional, because of course nobody else could see the reality behind the illusion. In fact, you yourself might suspect you're mentally ill (as Moning's heroine fears at first) until you meet others who share your frightening gift. A possible variation on the theme: What if you're one of the small minority who can see the aliens for what they are? But suppose you alone realize that the "invaders" aren't disguising themselves out of hostility, but out of fear. They have some good and innocent reason for hiding from the human population they're forced to live among. If you tried to convince your fellow "Sighted" minority of the aliens' innocence, most of them wouldn't believe you, and you would be doubly isolated.