Facial mimicry is a basic facet of social interaction, theorized to influence emotional contagion, rapport, and perception and interpretation of others' emotional facial expressions. Using EMG, two studies examined spontaneous mimicry of emotional facial expressions of live models over time, and whether the relationship between the model and observer moderated mimicry. Study 1 manipulated observers' liking of a confederate model; Study 2 compared friends with strangers. Observers mimicked brow and cheek movement. Observers who liked the models mimicked cheek movements more than those who did not like them. Study 2 demonstrated mimicry of natural, spontaneous expressions and found that observers' affect changed in association with the models'. People mimic live, dynamic facial expressions, likely more idiosyncratic and weaker than prototypes often used as stimuli, supporting the naturalistic importance of mimicry. Liking increases mimicry of smiles, indicating that mimicry is partially a consequence, not just a cause, of positive social relationships.