In particular, I'd like to highlight the parallel between an ongoing D&D campaign and a serialized TV show. Obviously with the PCs being the main characters a lot of the complex relationships will grow organically as the players interact amongst each other, thus making the comparison a lot less daunting for DMs! However, DMs still must think about this since the PCs aren't the only people populating the campaign world. Each "adventure" should feature at least a few fleshed out NPCs (more than just background NPCs, like the random shopkeeper or barmaid), and you should have a vague idea of these characters' personalities, motivations, and backstories. This obviously doesn't need to be too in-depth, but at least summarize each of these 3 traits in a sentence or 2. It doesn't have to be elaborate either; most NPCs are much more mundane than the Heroes!
For example, let's look at one of the main NPCs from my Talamhlar campaign, Gaknar (this campaign ended prematurely when I moved away for a seasonal job, and due to the months that passed before I came home we just never resumed it). To re-cap, that campaign started with the PCs captured in a castle, regaining consciousness with the place in chaos. A group of mages were doing various arcane experiments in the castle, including the creation of super-soldiers. Not surprisingly, the mages lost control of the super-soldiers, were largely massacred, and the magical containment was broken, freeing the PCs. In the room their was also a goblin named Gaknar, and he promised to show the PCs to where their equipment was and how to get out of the castle if they freed him (he'd been a servant for the mages). He accompanied the PCs throughout the campaign, becoming a valued member of the party (well, most of the PCs liked him). There were a few times where he acted somewhat suspiciously and was caught lying, and so was never 100% trusted. He even disappeared "on business" once, refusing to tell the PCs what the business was. So what was the deal with Gaknar, and what ideas did I have floating around in my head for him?
- Backstory: Gaknar was a runt in his tribe, always picked on for it, and some semblance of a conscience (moreso than most goblins, anyways) didn't help matters. During a raid he left his tribe, but finding it difficult to fend for himself ended up working for the mages.
- Personality: Sycophantic, and has quirky ways of showing affection. Sly and sneaky like most goblins, but with a greater sense of loyalty.
- Motivations/Secret: On his errands away from the mage's castle, Gaknar began to see what life was like in the civilized worlds. He became a spy for someone in the nearby town of Marblemount, helping to gather info on the mages to help shut them down.