Knowing that a character is the encoding of their emotions, thoughts, beliefs, and so on, in their author's mind, we can conclude that fanfiction writers cannot possibly have access to the character, since they cannot have direct access to the author's mind. Therefore, unless you talk to a character directly via the author (or the character switches with the author), you cannot interact with the original character at all.
The only thing you can do is make a "copy" of the character in your own mind, and interact with that copy. Usually, the "copy" will be different from the "copy" in the author's mind in many ways. In fact, the word "copy" isn't very accurate, because it's not possible to actual copy the character directly (at least, not with our current technology). Instead, all you can do is try to reconstruct a similar character based on what you know of the original.
Regardless, these "copies" are people. Treat them as you would any other person, and remember the 3 basic rules on how to treat characters:
Date: 2021-01-13
Author: Galene