Question: If I'm writing a story in which the narrator is actually not the protagonist, just a close friend/flatmate/love interest/etc (think Sherlock Holmes, except not about mysteries), would that change how I write it, or would it be pretty much the same, just not from the protagonists POV?
It depends who the main character is. And by that, I mean who's decision at the climax determines the outcome?
You can have a character narrator who is fairly uninvolved in the story and simply reports it. In that case, the main character may still be the protagonist and the narrator must convey the protagonist's point of view, including his internal conflict and the decision/action he takes at the crisis.
On the other hand, you can have a character narrator who is the main character. In this case, the character narrator will be wrestling with his/her own internal conflict. The narrator will have to make a decision at the climax of the story whether to change or stay the same, and that decision may determine the outcome of the story.
For instance, in the recent British TV series, Sherlock
(a 21st century retelling of Sherlock Holmes), Holmes is the impact character and Watson is the main character. Yet, if you watch the first episode, Holmes is also the protagonist pursuing the story goal of catching the murderer.
At the climax, Holmes confronts the murderer - a man who talks people into swallowing poison. For a moment, we are not sure who will win the conflict. It looks as though the murderer might talk Holmes into commiting suicide.
But Watson, an ex-soldier who hates civilian life, makes an important decision. He shoots the murderer and thereby chooses to become a man of action again. The resolution of his internal conflict also determines the outcome of the external story.
So you have to decide how interested you are in your narrator. How important will his internal conflict be? Who is the real main character? That will affect how you tell the story.