Assuming good faith

"Assume good faith" is one of the most fundamental principles we use on the Wikimedia wikis when we come across a contributor with whom we disagree. While it might seem an easy maxim to follow, it can and does become difficult when communications get fuzzy.

Questions

General:

1. When does assuming good faith come into play? What triggers us to make a conscious effort to do so?
2. Does AGF only come into play when we first encounter another editor? Is the assumption of good faith eventually replaced by earned trust (or earned distrust)?
3. Do we base the assumption of good faith soley on our own interactions with someone, or do we always take into account the user's history with others?

Difficulties:

1. How do we respond to a new user exhibiting "similar behavior patterns" as an old threat we've dealt with and learned that an assumption of good faith was incorrect?
2. How do we respond to users who claim to have redeemed themselves, after proving against assumptions of good faith?
3. How do we respond to people who, in good faith, don't assume good faith of us?