STRUCTURING CHARACTER ARCS - AND ISSUES BEYOND THE MAIN CHARACTER AND MAIN ARC 

 We can discuss big-picture structure but when we're talking about characters beyond the protagonist/hero/main character we're often at a loss. Frequently a clever move to make a story cohesive is to set all the characters on their own arcs that mirror each other. Often this is done with intersecting plots - B plots or subplots - featuring secondary characters - and they may be a way you can express the theme and examine it from other angles. Of course B plots should be connected to the core plot; they may converge to affect each other by the end or may intersect repeatedly over the course of the story, or may merely be separated but structurally strongly connected - but in any case, the plot threads should fit together in some way. 

One obvious secondary arc is the classic 'mirror' arc of an antagonist - a character who exemplifies the antithesis of the hero's values and may challenge them in various ways. The villain is a more flawed figure than the hero, and may be able to shine a light on what the hero could become unless they change. The villain might confront the hero directly or from a distance, might be violent or merely a rival in work or love, but the agenda of the hero and villain are usually antithetical and both cannot easily get what they want without the other losing it, perhaps because they want the same finite thing, or otherwise because they want two outcomes which are in some way fundamentally incompatible. 
 
This is where conflict emerges, and out of that conflict change must happen for the characters involved. There are some stories which lack a sentient rival; nature/wilderness or a large disaster in such cases may serve this purpose. Sometimes the rivalry is more subtle - as with a hero and secondary character who agree on the goal but disagree on what to do in pursuit of it. 
 
Relatability can be an interesting thing. Snyder's book 'Save the Cat' argues for the main character to be a likeable or 'good' character, albeit one with an imperfection which needs to be addressed. However, the advent of 'antiheroes' indicates that this is not necessary. A hero can be very deeply flawed, even a criminal or someone fairly evil, but the lesson to learn here is that the hero is pushed out of a comfort zone early in the story and faced with a challenge or obstacle that puts a lot at stake for them. The fact that they're reactive and that a wrong was done to them in some way, that they were threatened or thrust into a conflict, plus the fact that they are in some way interesting and have some sort of redeeming humanity in them, even if small, is usually enough to make the audience identify with them, especially when the alternative is even worse. 
 
Depth is also helpful in creating a unique character. Ask questions about your characters to add detail to them, and be prepared to dig a bit into each character, especially the major ones. Where and when did they grow up? What do they believe in? What are they good at? What's their career, what are their relationships with others? Family? Friends? Love Interests? What do they look like physically, and are they male/female? What are their flaws and vulnerabilities? Secrets, things they'd rather not discuss? Things they love to do - hobbies? Education level and experiences in the education system? Greatest fears or phobias? Any unusual skills or uncommon knowledge, or oddball traits? Any clubs, religious or social groups they attend? Who raised them, and what effect did that have on them? Do they have any children themselves? What are their big struggles in life, things they are really bad at, or perhaps health issues, work issues, relationship issues, etc? What are their political and ideological views? What major regrets, mistakes, traumas, or acheivements exist in the character's past? Are they optimistic, or cynical, somewhere in between? Introvert or extrovert? Who do they like or dislike, and why? 
The characters you build can, once, built, develop interactions with each other, and conflicts/tension, which are the core of any good drama. A Hollywood exec has sometimes said 'passion and tension' are the core of a compelling story. This is sometimes crassly interpreted as sex + violence, but really it is far broader. Passion is what each character likes or wants or cares about. Tension is a rivalry that grows out of the competing or conflicting passions of characters. Often the outcome one person wants, another doesn't, and this friction sets arguments and events in motion between a cast of characters, each with their individual traits [wants, and fears, usually merely the fear of losing something that one wants to have, losing one's life, one's friends, one's job, etc] and characters will act logically as an outgrowth of their own personality and attraction/aversion agenda, and the actions typically develop into webs or chains, in which one character does something, another responds, and the effects of each action trigger fallout and successive series of reactions. What happens to each character; the change in a character from the start to end of a story is that character's 'arc' - how their life is affected by the experiences and events that happen during the story. If no meaningful change occurs, there's no character growth, no drama, no dramatic arc. No real story. As Alfred Hitchcock once said, "Drama is life with the dull bits cut out". If nothing significant happens to anyone in your story, and it's boring, with no change of the situation from start to end, then it's a weak story. Your narrative, if it's to be entertaining, needs to be a significant moment in the lives of your characters, and not merely a nondescript day that is just like every other day in the characters' lives. Something momentous must be happening for them. Otherwise, why would it be worth writing about? 
 
A capable storyteller can even make us identify with those characters we'd normally want to hate. A good example would be the film Das Boot - a submarine thriller in which the heroes are drafted Nazi sailors trying to survive a war they have no hope of winning. Hopelessly outgunned by the Allied fleet, they overcome seemingly impossible odds to complete their arguably suicidal missions. We find ourselves accepting them as heroes even though they're Nazis, and have even killed Allied troops, because they clearly don't want to be there, because they're underdogs, because we can identify with the idea of them enduring miserable working conditions under duress, just wanting to have the war over so they can return home. 
In any other story, these'd be the bad guys. But when a storyteller finds some elements of authentic humanity in ostensibly unlikeable characters, that kind of story can work. 
 
Making us connect to the 'other' is a powerful ability that a skilled writer has; helping us to understand, to empathize, to care. Written or filmed narraties can be a powerful way to impact people emotionally and psychologically, and make them think and feel about things they wouldn't normally be considering. Which means it's a real shame that the mass media is dominated by the six big entertainment conglomerates that have essentially churning out 'marketable, uncontroversial product' like endless strings of superhero flicks, and generally not taking the opportunity to push any real storytelling boundaries. Defying convention is difficult when a company's business interests set the bulk of the Hollywood agenda instead of creative writers and directors. Worse, many of the directors now established continue making junk for years on end, and can continue doing so as long as enough of that junk turns a profit. (I'm looking at you, Michael Bay!) 
 
The good news is that there's a very real opening for indie video directors to do things that are fun and clever and truly imaginative. I won't say I'm any expert on the craft - I've mostly done comedies and sci-fi, fantasy, and action videos, and avoided heavy drama and the like - but there's absolutely a place for it on the indie scene. This said: I don't think the reactions against Hollywood by indies are always helpful to indies or their cause. Ideas like 'Dogma '95' - the refusal to have music scores, post-production beyond simple editing, etc - are a mistake. Visual effects and music scores are not what's wrong with Hollywood. They're tools that can, if used judiciously, enhance a story. Good visual effects work can allow 'what if' stories set in the future or other imagined realities which can be great fodder for approaching big philosophical ideas when used well, and in service of action sequences may increase excitement for the audience as well as allow big stunts to be done far more safely than would otherwise be possible. VFX are great for worldbuilding and expanding the range of stories we can tell. Music is great for enhancing emotional impact of those stories. 
 
I would not abandon either. However, I agree that it makes zero sense to cast 'name actors' in an indie simply because we can't afford to hire them! Better to find talent that can complete the scenes well, but which is not recognized widely. There are SO many struggling musicians and actors in the world, and there's a good case for hiring them because they'll work way cheaper than a 'name' and arguably could almost match the work of 'pros' in some cases. So those are some of my thoughts on video, not exactly what you wanted from this article but worth discussing I think.
The banner links at the bottom of each page include links to my own webpages, links to my storefronts/shops online, and occasional links to other [third-party] verifiably legitimate stores and services I've hand-picked, vetted, and used myself. 
That said, I'll openly disclose that when you buy something from another vendor using these links, for example, a link to Amazon.com, I will get a small fraction of the sale amount and will use that as a means of supporting and expanding the projects I'm working on here. These ads for other vendors, as well as - especially! - links to my own shops, are helping me keep these sites online and growing with new content on a more frequent basis.