Narrative is tricky thing even in the complete freedom of a short story or other written medium. It needs to be streamlined enough to allow a fluent story but also show the characters making choices that seem real within the structure of the narrative. Writing a continuation to a short story means taking these choices into account, how they influence the character’s psychological state and his future actions. It constrains the subsequent narrative. The benefits of a written story are that at least these choices are clearly written down on a page. Narrative becomes a different story when we look at a medium where this is not always true: video games.
The Mass Effect trilogy is an ambitious attempt at game storytelling. More than most other games the series makes the decision of players relevant to the narrative of the game itself. This has been done in other games, usually involving different endings based on taking a certain actions from a limited set of choices during the last act of the game, kind of like the “I should have done that earlier” realisation at the end of certain horror movies. The Mass Effect games are different, the player is aware that all actions have consequences. Write your own story. The problem: Every decision creates a new strand of the narrative which has to be represented in the rest of the game and all its successors.
Part One uses the decisions to its full advantage: choice of which major character to kill off? Sure! Fate of the galactic government? Be my guest! Kill of another major character with the wrong dialogue choice? Sounds dandy! It is of course streamlined, the narrative is still mostly linear in a messy multi-strand kind of way. But you can imagine the game designers having a blast. As a consequence the restrictions put on Part Two and Part Three are very substantial from both a narrative and an economical point of view. As either one of the major characters could have survived, both characters have to be presented in the next narrative. Which for a video game means twice the graphic art and voice acting. Subsequent, neither character is much present in Part Two and pretty restrained in Part Three. Too simplify things, the characters do and say very much the same things. Instead of well formed separate characters they became a single story device.
Which of course brings us to the ending controversy of Mass Effect 3. A lot of fans criticised it for a lack of true choice. While it is of course regrettable that the ending had to be streamlined and the final post decision scenes (pre-DLC) lacked a proper epilogue, there was not much choice for the game designer. From a game point of view, you are constrained by economics: it’s hard to justify that much extra development work for disparate endings. From a storytelling point I can understand it as well, you want to finish an epic allowing all gamers to experience a similar conclusion instead of dealing with story strands drifting apart.
Long story short, games allow storytellers the choice to give choices but it might dramatically restrain the story itself in the long run.