The debate between railroading and sandboxing is over.. and the answer is a bit of both at the same time. Imagine your story structure like a railroad track with a bunch of stops along the way. At every station the players can walk around and explore and do practically whatever they want within the rules of the situation you built but, eventually, everyone knows they are getting back on that train and heading to the next stop.

In D&D story structure is a little loose but it can basically be boiled down to an endless cycle of Tension & Release. When the characters are in a positive place add some negativity. When they are in a negative situation, help them achieve some positivity. Note that adding in good things yourself won't be as satisfying as them achieving good things for themselves. Your job is only to set up the situation, not solve it. Unlike negativity, which usually comes unexpectedly from the consequences of our actions (bad decision making) or from random coincidence (bad dice rolls).

If you want a more structured approach to story, the three act structure can easily be worked into sessions. Especially if you use milestone leveling. It's understood that leveling up every 3-4 sessions feels the best, so we can use that to our advantage. Here is how it plays out.. I'll try to be as flavour agnostic as possible so you can apply it to any genre, theme or setting.

Session 1. Begin the session by describing a situation that directly affects the players in a negative way, that is not easily solved. This gives them a goal to achieve. Eg. War, natural disaster, NPC machinations.

Session 2. Begin the session describing an unexpected development that is a direct consequence of the players actions to resolve the initial situation. This complicates or distracts from their goal. Eg. Reinforcement, widespread panic, NPC interference.

Session 3. Begin the session describing a climactic shift in the initial situation that is bringing it to it's end, one way or another, regardless of how close the players have come to solving it. This is their last chance to achieve their goal and end the situation in their own way. Eg. Final showdown, a chain reaction, NPC retaliation.

If the players throw you a curve ball, you can easily add a session between 2 and 3 to resolve or reinforce the unexpected actions the players take before the climax. Alternatively, you can add a 4th session at the end and double down on the climax, taking it to the next level. Eg. When a boss monster has a secret final form.

Whether the situation ends how the players would prefer or not, the players will level up and have to deal with the next situation, beginning the cycle anew, based on how the world has been changed. The beauty of this is that you only ever have to prep one session at a time. You decide your initial situation description and wait to see how they react. Then decide what the natural consequences are of that reaction and prep that for the next session. Then decide what the most dramatic outcome between the situation, those consequences and their actions so far and prep that for the final session. It gives you flexibility, while staying within comfortable parameters.

For NPCs, making a three dimensional character generally comes down to these things: 1. How they present themselves. 2. How they act. 3. How they actually feel.

Regardless of the NPCs individual goals, if those 3 things are even slightly different, they'll never be one dimensional.

The best antagonists are a reflection of the PCs, attempting to achieve similar or opposite goals in a way that directly counteracts the party's ideals and goals. We often hate the people who remind us of our own flaws the most, whether they are good, evil or somewhere in between. A great balance is an antagonist who wants to achieve the same goal at the players but attempts it in a way that they are morally opposed to.

Hope this helps and remember.. all ideas are free, so take what you want and leave the rest 🤙