We often unconsciously mirror the behavior of people we interact with. This can include mirroring posture, gestures, and voice patterns. A recent paper in Current Biology reports that we can mirror a smile based on speech alone, and even do so without actually detecting the smile.
The researchers applied a signal processing technique for altering recorded speech under a neutral mouth position to what it would have sounded like had the speaker been smiling. They played 60 such recordings (some manipulated, some not) to 35 subjects, and asked them to judge whether the speaker was smiling. The researchers also measured the responses of two subject muscle groups while listening, the zygomatic (smiling) muscles and the corrugator (frowning) muscles.
When the subjects correctly reported neutral expression or smiling in the speech, both of their muscle groups accurately mirrored the speech while listening (e.g., for smiling speakers, zygomatic tensing and corrugator relaxing). Interestingly, even when the subjects were wrong, their zygomatic muscles still mirrored correctly. This was not true for the corrugators, which instead reflected the subjects' report.
Our mirroring capabilities go well beyond what we see, or even perceive.