What Resting Face Is (and Isn't)
A resting face is the face you make when you are not deliberately expressing anything, the default configuration of your facial muscles in a relaxed state. Despite the term 'neutral', resting faces are not expressionless. Small variations in the natural resting tension of facial muscles, and in the structure of features like the mouth corners and brow, make some faces appear happy, sad, or stern even at rest.
These resting face variations reflect genuine structural differences between individuals, not expressions, but the baseline geometry of the face in an unexerted state. Face recognition systems process these structural differences as identity signals, which is appropriate: your resting face geometry is part of who you are.
Why Neutral Expression Produces the Best Matches
Strong expressions, smiling, frowning, raising eyebrows, temporarily alter the apparent positions of facial landmarks. A wide smile raises the cheeks, partially closes the eyes, and stretches the mouth horizontally. These changes shift the facial embedding, because the apparent geometry of the face differs from its neutral-state configuration.
Celebrity training data includes varied expressions, so the system handles expression variation reasonably well. But using a photo with a neutral or mildly positive expression produces the most stable, accurate embedding. Strong expressions add noise to the geometric representation, potentially moving the embedding away from the position it occupies in your neutral-face photos.