Personally, I like to look at the main non-player characters, or non-player interest groups. Main npc's can be the local king, baron or warlord. Or a powerful wizard, tradesman, bishop or even a monster – preferably an intelligent species. Interest groups can be the trade guild, a gang of slavers, the city guard, the kings guard, a mercenary war band, an organised crime group, the church, town hall, an industrial cartel, a hidden organisation of priestesses, a band of cultists, the secret police, and so on.
Whenever players act (or don't act), I try to think: what will the npc's and interest groups do? Will they notice the player characters? What will they do about them? Will they contact them, be pleased and go at their own business, be angered, annoyed, shocked? Will they send in assassins, or instead send a head hunter to hire them, or seek an alliance?
Take It One Group at a Time
Try to start out with just a few npcs and interest groups, and move them about on the imaginary playing board of your world. Make sure that at least a few will at some time turn out as enemies, or allies to the player characters. Make them have conflicting interests, also with eachother. And figure what the players might see, hear and notice of the ongoing conflicts, dealings and secret alliances.
Maybe the players will witness how a slaver band captures the daughter of the innkeeper (actually a mistake). Then they see how the city guard stands idly by (they have a conflict with the innkeeper, and one city guard works with the slavers). The heroes may protest with the mayor about the guard, and the mayor may then reprimand the guards. Next the heroes are hired by the innkeeper to rescue his daughter. The one city guard in league with the slavers sneaks out at night to meet with his fellows (and might be followed by a hero). The slavers meanwhile realize their mistake, and make plans to get rid of the daughter (the heroes may be witness, or they may learn later). And so on.
What if You Forgot Someone?
Sometimes you'll find that you forgot about an npc or interest group. Players may ask you why they didn't do anything. In that case, you may do several things – besides frankly admitting that you forgot. One is to say that they were too occupied to intervene or act, or that they had missed what happened – npcs can surely make mistakes too. Another is to smile enigmatically, and think of a cooler reason why the interest group didn't turn up. Maybe they have a secret friendship, an ulterior motive, or maybe they are scared of something or someone that was present. Maybe they care less than they seemingly did. Or maybe the players received help from a third party, who intervened and stopped the missing npc or group. But I didn't have to say so, did I? Because you already thought that up beforehand, and that's why they were not there... That's what the players will find out next time. That's the secret of the Game Master.