Prep situations. Prep NPCs with goals and motivations, give them plans. Create situations rife with conflict, preferably ones where there is nuance and who is right and who is wrong is not obvious. Include elements the characters might care about or which speak to their values and desires. Once the board is set* introduce the character in the mix, and play the situation and NPCs with honestly, expressing their nature as genuinely as you can.
* A metaphorical board.
The above is very close to the standard Narrativist model for situational play but I want to emphasises a few things.
One big difference is that you almost always want to start with, at the very least, an implied connection between the agendas of the player characters and the NPC's. In other words, having an adventuring party roll into town tends to produce weak sauce. I mean it can work as long as everyone is genuinely and actually committed to nothing happening. No saying one thing 'we have the agency to not bite hooks' and yet doing another 'well there would be no game it we didn't bite, It would be rude to the GM to waste their work, and so on.' For anyone serious about this style I'd actually kill the whole idea of adventures, parties and plot hooks. Think character centric drama instead.
There is a point at which the situation is established. Normally before play but sometimes bleeding into the beginning of play. THE situation (singular) is the basic unit of play and the aim of play is to resolve it. This could take one session or many sessions. In a game like In a Wicked Age, you're limited to one session. In a game like Sorcerer you don't know how many it could take because this depends on the actions of the characters. Could be one or two or several.
Sometimes there's a broader situational question (or several) that carries on through the established situation. In Dogs in the Vineyard, each session involves the Dogs going into a town and sorting out it's troubles. This is the basic unit. Yet this experience changes the individual Dogs and their relationship to their faith. So in the next situation, we'll all see how that resolves or changes.
In a Wicked Age can have a similar dynamic. Each situation new characters are either generated or return. The situation is then played to resolution. If there are hanging threads or unanswered questions, these can become part of the next situation.
What makes this different to play that may 'seem' similar, is that we really mean it when we say the situation is the basic unit. The GM isn't introducing new stuff for purposes of pacing or to try and resolve threads. To do so is to put their hand on the scales and essentially give themselves story control. As
@pemerton mentioned earlier, the atomic components are the various characters. Therefore be very wary when introducing new characters.
Usually the introduction of new characters happens via two related routes. A character already involved in the situation does something that causes someone(s) implied by the wider setting to enter the situation. For instance a murder in a small town causes the Sheriff to become involved. A rebellion in a colony outpost means a battalion of Imperial troops (and their commander) are despatched.
Minor NPC's, such as those implied by a group of people. End up becoming more important than they initially seem. For instance, a captured soldier is convinced by a player that their cause is unjust. At this point you'll have to think about who that character is and what their priorities now are. As a general rule you're doing this in response to wanting to see the situation as it exists resolve, not adding crazy stuff because it shakes things up.