Steps
Observe body language
Notice their posture - how they hold their shoulders and head, and their gestures - how they move their hands, head, and face
Match movement patterns
Approximate how they hold and move their body, including the speed of their gestures and whether they're expressive or reserved
Mirror eye contact style
Notice whether they make direct eye contact frequently or look away, and match their general pattern
Sync voice characteristics
Listen to their voice tone and speed, then match the general pace and whether they speak musically or more monotone
Capture the spirit
Focus on getting the spirit of their physical and vocal presence rather than exact imitation to avoid seeming like you're mocking them