Eyes
Heinz played the part of a villain, but Perry knew him better. Behind those deep blue eyes was pain. Deep, emotional pain that Perry knew was caused by the man's horrible childhood.
Perry played the part of a fierce and determined secret agent. Heinz could see in those cocoa-brown eyes that it was true. But, there was something else, too. A small sense of...longing? As though he was keeping secrets from someone.
A person's eyes can tell you a lot about him. This was certainly true in their case.