That's right -- subjects were unable to anticipate a fake smile. Also, when one subject gave a genuine smile, they got a genuine one in return. When their smile was fake, so was the responding smile. In other words, a genuine smile and a fake smile -- though identical on the surface -- are treated as completely different expressions by the brain. Likewise, a separate study found that we're not actually fooled by fake laughter, either -- on some level your uncle knows his racist jokes are only getting polite fake laughs, but he just doesn't care.
"We've never showed genuine emotion at Thanksgiving so far. I figured, why start now?"
You might find yourself wondering why we bother faking smiles at all. Why maintain this fiction that pleasant feelings can be faked? Why not just be up front and let our expressions reflect what we're feeling inside? It's because once you start asking questions like that, all of the rules of polite society will come crashing down. A sudden wave of total honesty would be the one disaster our civilization could not endure.