Typically with LRP you’re putting your players into a situation they have to react to. Whether they’re playing good, bad or (most often) some kind of grey, there will be a circumstance (or circumstances) that they will want to change as part of the game. Ideally you'll have players with different ideas as to what should be done. Some of them may be of the opinion that they should be stopping other players changing things. There will be conflict.
In Empire they have to fight off a threat to the Empire. They’re defending the lands they see as theirs. That’s the main threat. There’s also the internal politics. People compete for control of the political situation. The compete to make decisions go their way. There are other threads, but each is a situation that requires players to influence it in order to achieve their goals.
In modern LRP there are fewer true monsters. We are starting to include the idea of causes and effects. We can see what created a behavior or a problem. Sometimes you want something to just be bad. Other times there's a little more flexibility there. You want your players to doubt themselves. We often play with emotions. Using small delicate people as something evil tends to play with peoples expectations. Making someone who is generally pretty together appear vulnerable also makes people have to think about what they’re doing.
A lot of good plots ask players to balance several sensitive areas. By giving them several conflicting wants and needs, and not enough resources to make everyone happy you make them make choices. These are typically good choices because your players aren't trying to guess what the ref wants. They're trying to work out what they're willing to sacrifice, or what they live without. Whether they'd rather steal something in order to get what they need or if they're happier leaving the poor children hungry. Which of these is less of a moral compromise. Obviously this doesn't work if they're playing shades of grey characters, but with a mixed party it can lead to interesting turning a blind eye situations.
It’s a general rule that players will talk to the things you want them to hit and hit the things you expect them to talk to, and that this will happen whether or not you take this rule into consideration. It’s also true that the thing you absolutely do not want is your players going to interact with something and pausing to think “did they mean me to hit this or talk to it?” and you will get that. What you really are aiming for is players that will react naturally to whatever situation is thrown at them, and just as in real life that can be a wee bit surprising.
Any creature the players encounter should make sense. Even if the players never find out why there should be a reason for every alien to be where it is, for each villager to be where it is, and if it is something the players can talk to the person playing it should know what that reason is if it makes sense that they would do so.
My favourite games are balances. With Alone games we typically built in an escape route for about 6-8 players. Most players were going to die. There was no way out. Alone was set up to finish with a victorious death. This got very interesting in the tunnels. To keep the atmosphere we'd said they couldn't have clocks and hand crank torches only. It kept low light and meant they didn't know what the time was. One group concluded they were getting close to midnight on saturday night and that they would have to complete their objectives. They had a beautiful suicide scene. It was glorious. Unfortunately it was only about 4pm. They seemed happy with it. They knew they were very unlikely to survive and did something epic. They also felt they'd had their game. (I only know this one from other people telling me, so accuracy is down to my notoriously dodgy memory).
With the Alone games we didn't give everyone enough stuff to complete the survival objective. The joy was in the game, and everyone surviving wasn't ever that high on our list of important things. It was about putting them in an environment and letting them interact. We had a few ways they could get the things they needed and a whole lot of choices about who to trust, who to shoot and who to secretly plant alien embryos in in order that you can sneak them back to your parent corp and sell them for money. It didn't matter what they spoke to and what they shot (although shooting the AI and talking to the alien was a fast route to a confusing death).
The world you are creating needs to have some form of back story, and to hang together. It gives everyone the common threads to explain why the Montagues hate the Capulets, and helps players to see the effect their having on the world. After all, they're the god damn heroes, whether or not they all end up dead.