Since Dusk City Outlaws is based on sandbox-style play, and knowing that I wanted the game to be one that required a minimal amount of prep work to start playing, I knew that I wouldn't be able to include a lot of plot or narrative in the scenarios, and would need a mechanic in the game that carried the weight of providing dramatic twists. That mechanic is called heat, and it represents the severity of the reaction of the city's law-abiding citizens to the criminal activities of the cartels. Heat is also a resource that the Judge spends to create impromptu challenges that the members of the crew need to deal with in order to succeed.
Lots of thing generate heat; just by being on the job, the crew generates heat, representing the ever-present pursuit of the Crown's investigators and the officers of the City Watch. The players can also generate more heat by being sloppy or unlucky when committing crimes: leaving behind lots of evidence, stealing objects of exceptional value, causing collateral damage, killing or kidnapping someone, etc. Crew members generate a small amount of heat by being conspicuous in certain areas of the city, though they do not generate heat for most crimes by default, only when the crimes are egregious or reckless enough to draw the attention of the Crown. Heat can also be generated as a negative consequence of rolling drawbacks on the game's challenge dice.
The Judge spends heat to introduce complications during legwork scenes, or when the climactic scene of the scenario is going down. For small amounts of heat, the Judge can add private security guards into a scene on the fly, introduce hazards and obstacles that pose a physical threat, or cause a mark to become exceptionally suspicious for one scene. Greater expenditures of heat can have a bigger effect: a district of the city is plastered with wanted posters featuring images of the crew, the citizens of a district grow suspicious and are less susceptible to being fooled, or a more significant antagonist, like an investigator or bounty hunter, enters the scene.
When the heat pool grows large enough, the Judge can spend a large chunk of it all at once to introduce a plot twist. Plot twists are major shifts in the scenario that might recontextualize the entire affair. A plot twist could reveal that one of the crew's allies is actually secretly an agent of the Crown who has been spying on them, or it could cause the City Watch to scoop up that ally in a raid and imprison him or her in the Castle, far from where they can be of assistance. A plot twist can introduce a major villain, like a member of the city's secret police or a Dredger investigator, or it could be used to place an entire district of the city on lockdown, making it impossible to move in and out of that district without having to deal with the City Watch.
The Judge makes the call as to the nature of the complication or plot twist that gets introduced. It always needs to make sense in the context of the scenario's narrative, and some scenarios come with special plot twist suggestions specific to the circumstances of that particular Job. This also allows players to replay previously-played scenarios, as plot twists and complications can drastically alter the way the Job plays out. Heat reduces the need for advanced prep work by allowing the Judge to read the current situation and make things more interesting on the fly instead of requiring the Judge to follow a script laid out in the scenario.