So I just got started on my first piece and iv already hit somewhat of a snag. It's going to be a story about a cop who is trying to find his partner. His partner will be disappearing in the first scenes but is with the MC in the very beginning. So my question is how thoroughly should I develop the partner as a character? Would it still be believable that the MC is willing to go through terrible trials to save him if their personal relationship isn't touched upon? I want to establish who the MC is very quickly (while on their way to a call) so I can get the reader to the part where the partner disappears thus setting the tone for the rest of the story.
We'll care about the partner only as much as the MC does, and as you're able to create that caring (by creating empathy with the MC, so we care about what he cares about). It's not a matter of the partner even getting any screen time at all, but in how well and genuine you establish the MC's perspective. If you're writing this story, and their personal relationship isn't touched upon by essentially every action in every scene, even if the partner isn't present, then the story will fail, as it won't be about a cop trying to find the partner he cares about anymore. Don't write about events or plots, write about people and relationships.
I will preface my comment, I do not know how it works in "fringe fiction," my thoughts directed only toward the main stream. What you actually write in so far as that relationship is up to you. However as a creator, knowing the dynamics between the two, book, chapter and verse will spawn a more authentic reading work. Do not be afraid to waste a few words flash fictioning how they acted/reacted, related and/or did not relate to the each other, if you use it in a final draft great, if you do not it still will not be a loss. In so far as the trend in the last thirty years, relationships between people has become what the general reading public demands. Suffice to say The Mr Roakre type characters will be getting few calls to the big leagues. A stellar air tight plot even one written with Steinbeck "tone" that has none of the "boring" (says who it's boring?) "people stuff" is a crash course for failure. As it relates to the missing partner, I would write is as if they were not the best of friends, I would sprinkle in some distain for each other but in my world there are few shiny, happy perfect relationships
I'd throw in a few skeletons in the partner's closet that your MC has no idea about but discovers at key points in the story. I like reading stories where the relationship between characters is revealed gradually.
Even better, when he does discover the "skeletons", the MC still continues to work and try to save his partner. That will really help.