When contact is first made between two centres of empathy located in two human brains, there is an exchange of protocols that is referred to by neuropsychologists as "handshaking." This term derives from IT. When two entities in a system - for instance a computer and a printer - are about to exchange data, they first negotiate to be sure each is ready to transmit and receive data. This preliminary negotiation is also referred to as "handshaking," which term of course derives from the real thing.
From a practical acting point of view, this initial contact, usually through the eyes, makes the difference between treating your fellow actor as an object or as a human being. Understanding how to find it is essential for building trust and complicity.
Interesting that this effect has a scientific, neurological basis.