There's different situations that produce very different reactions. When the encounters are initiated by humans encroaching - accidentally or deliberately - on their home turf and they give the witnesses an eviction notice, things get serious in a hurry. Been there, done that. To stick around and disregard their warning is not only foolhardy but dangerous.
But, when they come into plain sight after a person (usually doesn't work with a group) has peacefully & openly tried to interact with them, it's totally a different encounter. The second two that I saw deliberately showed themselves briefly just to let me know they were not afraid of me. (Just as all the monkeys I've interacted with in Brazil, BF does not forget.) If treated respectfully, they will react in kind. If mistreated or insulted by someone, they will eventually get the chance to get even,