“Giles: A, a Slayer slays, a Watcher...
Watchers train and prepare vampire Slayers for their fated tasks. They are the Slayers’ contact with the Council. But more than that, Watchers are supposed to also teach the Chosen Ones what they need to learn in life, as a father would.
The Watcher must teach his o her Slayer to accept that they will not always win and that people will inevitably die. Life is not a fairy tale and most of the time happy endings do not occur.
As with the Slayer herself, if a Watcher perishes, the next one will be called. However, Watchers are fully aware of their role since they have use of reason and memory. They study all their lives and are eventually chosen by the Council to approach the Slayer. It is common practice that if a Watcher is to be replaced, the new one must come from a different family.
Who becomes a Watcher is determined by bloodlines. Offspring of previous members may become Watchers as well, and in most cases, they must.
They are researchers. Their duties rely heavily on the study of not just lore and prophecies, but previous Watcher’s journals as well.
As every Slayer is indeed different, their Watcher must adapt to her, not the other way around. Their job is not to force or demand anything from the Slayer, but rather to instruct and guide.