Not all stories are released all at once, and instead come out episode by episode, or chapter by chapter. This rather obvious statement has profound ramifications for fictional characters, who are formed entirely by the passion of the audience with their knowledge of the canon.
Because the audience is a pivotal force in making fictional characters exist, they are composed of the audiences interpretations. This means that when an episode is released, the characters have the same knowledge as the audience does up to that point. They do not know of the plans of authors for future developments or things that have technically already happened in the past but haven't been revealed yet. All characters are more or less at the part of their character arcs when the latest episode released.
Characters who have only been revealed to the audience in teasers are only semi-aware, similar to characters with a deficit of canon. The first episode or chapter releasing would be considered the story's real birthday, even if they were technically shared with an audience before that. After becoming fully formed characters, any teasers may be experienced more like ominous prophetic visions, either during waking life or in dreams.
A character from a canceled story has to spend the rest of their existence wondering what might have been and if they'll ever get closure. Usually they seek professionals to help with this experience, commonly to predict how the story would have ended had it been completed. It is still possible to live a fulfilling existence as a whole character even when from an incomplete work – many old characters have existed that way for centuries – but it is an inherently different experience from most characters so cancelation or death of the author during production are still widely feared fates.