As I've mentioned before, I'm working on a military sci-fi novel.
Here's the trouble with the military: you don't spend all of your service, start to finish, with the same people. Not all the people you've done Basic Training with will proceed to the same Advanced Training as you. Not everyone who completes Advanced Training with you will be assigned to the same unit as you. In effect, after each transition, one is meeting new people and making new friends, keeping in touch with only a small sub-group of the friends from before. Like transitioning from middle school to high school, and from high school to university, only on a significantly shorter time frame. And that's before I so much as touch on drop-outs (and I do need those for tension - it could, theoretically happen to the MC).
The result of the above is I'm asking the reader to get to know a set of characters, only to lose sight of them several chapters later. The characters the MC was closest to in each stage do get further involvement in the story, but most drop out of sight. Similarly, in Victor Hugo's Les Misérables, we don't hear about Bishop Myriel, his sister and his servant after their scene with Jean Valjean, nor about Félix Tholomyès and his friends after he abandons Fantine. Only, I can't think of more modern examples, which troubles me.
I would like to give my first-quarter-of-the-novel characters as much loving attention as Hugo gives Myriel and Tholomyès (and it's the recommendation I receive here), but I fear to lose the more impatient modern reader's attention. Much like the MC, I expect my reader would be eager to get out of boot camp and into real battle. How do I balance making the boot camp part interesting by way of having the MC develop various relationships with well-rounded characters, against the fact that most of those characters disappear from the story after the MC leaves boot camp?