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What @linksassin says is a good approach, but it's not necessarily the only approach. Sometimes, one medium is the "main story", while the others are "supplementary material". This is the approach ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/43034 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/43034 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
What @linksassin says is a good approach, but it's not necessarily the only approach. Sometimes, **one medium is the "main story", while the others are "supplementary material"**. This is the approach taken by Bioware, and several other strong videogame companies: the main story is in the games, while comics and books serve as tie-ins. They are not necessary for the understanding of the games, they get Easter-egg mention in the main games. But they do not stand on their own either. If you wish, they serve as "merchandise" for the games, and keep interest up while the next game is in development. With other games, most of the story is in the books, while the games focus solely on play. The story is a nice extra, but it is not necessary for gameplay. Nonetheless, the game is still the main product, while the stories are "merchandise" for game fans. On the other hand, the _Harry Potter_ videogames rely on familiarity with the books or the films, while the _Star Trek_ books and videogames rely on the TV series. In all those examples, **the story has achieved sufficient popularity in its primary medium, that it can rely on part of a strong fanbase to branch into other media as well, generating more profit.**