The critical thing to understand about telling a story is that you are not relating an entire series of events. You are not telling everything that happened as you would in a police report.
You are not drawing a line.
You are plotting a series of points, along that line, as a sort of connect-the-dots puzzle, allowing the reader's brain to infer the boring parts with minimum effort, while focusing on the interesting bits.
And just what the dots, the interesting bits, are and aren't depends on what sort of story it is.
Characters in Lord of the Rings never think or talk about sex, despite George Martin's objections, because it isn't that kind of story. It's the story of a existential struggle against a force of pure evil. Libido simply isn't relevant.
In Theft of Fire, characters think about sex a great deal, not because I share Martin's halfwitted objections to Tolkien, but because it IS that kind of story... the whole intention is for certain characters to begin the story as creatures ruled by their impulses and appetites, and then to develop.
However, no one in Theft of Fire is seen to eat, beyond Miranda taking one bite of sandwich, which is actually a power play, not a meal. Why? Because it's not relevant.
Characters in Redwall and Song of Ice and Fire have sumptuous feasts, because it is.
Any event that isn't in the furtherance or fulfillment of some story promise is BORING, and should be left in behind the dots, for the reader to infer.
It's unimportant whether Sam is having sexual fantasies about Rosie while journeying through Mordor. We just need to know that he misses her. We don't need to know every detail of Marcus and Miranda's conversations in a solid month of stealthily gliding towards the outer solar system. We just need to be able to infer that they are alternating between arguments and very subtle flirtation.
If you stick to the right dots, and don't put them in the wrong points, or try to draw the whole curve, then the story stays interesting.
The challenge here, however, is not only understanding that we are drawing the connect-the-dots puzzle, writing only the high points, but in knowing what our story actually is, what the readers care about, what promises we have made.
The story is established by promises made to readers, which we must fulfill. These promises are not about what will happen, but of what, and who, the story will be about.
The danger here is that the more popular, lengthy, and detailed the series is, the more easy it is for what authors are interested in to diverge from what readers are interested in. Then they scamper off drawing irrelevant dots, delaying or even avoiding story developments that readers are invested in and waiting for.
One important reason this can happen is that authors spend a lot more time with our characters and settings than readers do.
If I publish one Orbital Space novel a year, then every year, readers spend about twenty hours with Marcus, Miranda, and Leela. Forty if they re-read when the sequel is impending.
But if I spend 8 hours a day, 6 days a week, working on that sequel, then I am spending 2500 hours with those same characters. This doesn't necessarily mean I get bored of them, but it does mean I have a lot of time to get interested in other characters and secondary aspects of the story.
If I don't understand the gap between my interests and the readers', then I am in danger of boring the reader by drawing dots on these things for their own sake, either without first setting up promises that make the reader care, or at least without doing so adequately.
The more of a celebrity an author is, the more he sells, and the longer his career becomes, the greater the danger of this. Once a creator becomes interested in things his audience isn't, it's up to his support staff to open his eyes and get him back on track. But if he is viewed as The Great One, and has no one with the balls and job security to tell him he is full of shit and this chapter sucks, it will take great strength of character to avoid disappearing up his own ass.
This is why the middle books of Wheel of Time start to drag. This is why Rhythm of War was half again as long as it needed to be, full of irrelevant viewpoints that you suffered through while you waited to return to the story you were invested in.
And this is why the Star Wars prequels sucked.
No one was willing to argue with The Great One.
If you want your readers to be interested in new things that weren't promised in the first volume, you have to make new promises, and make them every bit as carefully as you did the original ones.
A secondary character cannot become a primary character without specific, intentional, additional work to make them so. Their prior role as a secondary character will not suffice. You must do more than that.
And a story only has room for so many primary characters, because the reader only has so many fucks to give.
Don't spread them too thin.
Personally, I would not introduce a new primary character after the first volume unless the story absolutely requires it, and then I would spend the same effort I spent setting up the original story.
And I'm careful to keep someone around whose specific role is to argue with me.
Her name is Natalie. To Natalie, I will never be The Great One, no matter how many books I sell, or get made into movies. Because she knew me before I wrote them, and I will always be that guy who married her sister, the one with the knack for storytelling. Talented, but not an object of veneration.
She was my first beta reader, and she devoured the book in the course of a single weekend, then threatened to strangle me because I hadn't included enough of a celebratory moment at the end. She utterly lacks the capability to carry out such threats, but I could see she was right.
Being a professional storyteller requires a careful blend of creative rule-breaking and strict self-discipline.
You have to love the stories you tell, but you also must remember they are not for you, but for others. You have to reliably fulfill the promises you made, without being predictable enough to let the audience guess how you will do it.
And most of all, you have to know where to put the dots.