Interference is terminology introduced in the series to denote a person who enters a video game universe. In the case of The Interference series, this term also translates to the 'self-insert' concept (a type of fanfiction where an author inserts themselves into the story, though not necessarily the canon universe or as the main character). The same word also refers to the event of entering the game world.
Interfering[]
When an Interference begins, a person's heart connects with the world of a video game, generally at the beginning. Once the connection is made, the relevant display device glows immensely until the light fully encompasses the person.
During an Interference, part of their heart's light is used to construct the game's world to a high degree of accuracy - though not exact; when Alexander Karsath first interferes in Kingdom Hearts II, he discovers that the voices of the Hollow Bastion Restoration Committee are the same as the actors of the first game rather than their actual voices at that point, implying that a person can influence the details of the world they build on a very small scale. Despite this influence, a game world will retain its overall chronology, including future events in its timeline that have not been released at the time of the event. As building a world takes an excessive amount of light, the individual may be left with a large amount of darkness in their hearts.
Once the world-building is completed, the person is then transferred into the game world, and wake up at the section of the game their heart connected. After this point, an Interference is free to interact with the game's characters and settings in any way they wish.
Because of the extensive and draining requirements of normal Interferences, it is clear that a person needs to be in possession of an extremely strong heart in order to enter. However, of Interfering in a world that has previously been Interfered in, then the Interference need pay no light, as there is no need to reconstruct it.
Universal Failsafe Law[]
During an Interference, the real world is subject to something that Joumae refers to as the Universal Failsafe Law. According to Joumae's description in The Interference, the world's time freezes to allow the Interference to make their journey and return without arousing the suspicion of non-Interferences. The effect also applies in reverse to game worlds - the world is frozen in relation to the time spent in it to allow reality time to 'catch up'.
During The Interference II, when Erica Karsath interferes with Fullmetal Alchemist 2: Curse of the Crimson Elixir, it is then revealed that items or persons that have entered the game worlds are not frozen with the rest of the real world while an Interference is taking place. The rest of the world takes on a greyscale look to indicate that it is frozen.
Another part of the apparent Failsafe Law makes itself evident at the end of The Interference III. When Alex returns Edward and Alphonse Elric to Amestris, it appears that no time has elapsed since Amestris was left. This may indicate that the Failsafe Law also applies internally, but this is not depicted entirely consistently. Whereas Edward and Alphonse leaving their world is demonstrated to stop time for Amestris in The Interference III, in The Interference IV, the worlds outside of Haven City un-freeze after the two-year prologue, with Even able to visit Haven nearly an entire year earlier through unknown methods. In Re:coded and Trials of Remembrance, the Kingdom Hearts universe is then shown to be moving normally, after the two-year time-freeze has ended and despite occupants of the Kingdom Hearts universe being in another world. (However, as the Kingdom Hearts universe technically serves as a hub for the rest of the universes, it may be that different rules apply to it.)
Finally, according to Joumae in The Interference IV, if an Interference dies in the game universe, they are wiped from reality, as is their history (and, theorised by Joumae, even the act leading to the moment of conception). Joumae offers this as an explanation for why Erica seems to have stopped existing in the real-world. He mentions a related phenomenon of Haven City disappearing when dissuading Even from stabbing Alex with the Kingdom Copy. It is implied that the erasure of Haven City would be part of the process of wiping all traces of the Interference from reality, and that the only thing stopping Amestris from being destroyed was Alex's presence inside at the time. Of course, it is worth a mention that Erica did not die, she was only transformed into a Heartless, and that the Kingdom Copy would likely have done the same thing to Alex; it is possible, though, that Heartless transformation or destruction is considered the same as death by the rules.
List of Interferences[]
Alexander Karsath[]
The series protagonist and the character with the most interferences under their belt, creating three of the four known worlds featured so far. Alex tries his hardest to keep the plot on the rails as much as possible, and occasionally strikes out on his own to make changes, avoiding any division of control over the situation; consequently, he is thrown out of his comfort zone when something happens outside of his clearly-defined plan. He tends to be quite well-researched on each of the games he interferes with, and so is often referenced as a source of information, which he personally finds to be the most valuable asset.
Erica Karsath[]
Alex's younger sister. She performs only one interference, creating the world of Amestris in the process. Her interference was apparently a completely separate world until Alex linked the two. Erica's efforts after interfering are considered slipshod by her older brother - for example, she frequently shouts information while expecting the characters to simply listen to her. Her status as being a fan of the Fullmetal Alchemist franchise and Edward in particular, rather than the actual game she interferes in, leaves her in a position where she cannot answer major questions relating to the game's plot.
Differences in Interferences[]
Alex and Erica's interferences are slightly different, most likely due to both the natures of the games they interfere and because they come from the same household:
In Alex's Interference, a facsimile of Alex's family's apartment is transferred into the apparent corresponding position of Roxas' apartment in the Virtual Twilight Town, the place where Kingdom Hearts II begins. It is also stated that Alex has been given an alibi in this world as a new tenant of the previously unoccupied apartment, and the universe makes a space for him by giving him the Lockshield as well. Alex keeps all his own clothing; a sequence later depicts Sora receiving new travelling clothes, which is when Alex receives his enchanted clothing.
In Erica's Interference, however, Erica is simply dropped into the middle of the desert directly in front of Edward and Alphonse. This is where the game starts, and may also be because the Karsath apartment already inhabits another space in the game worlds. She is given no alibi and no weapon. She is also instantly given a set of travelling clothes. This may be because unlike in Kingdom Hearts II, where Sora is given new clothing, there is no such change of uniform for the Elric brothers that would offer the same in-universe protections.