Of course the problem arises: With millions of people to control, how exactly do you keep track of all individual people? Even a government cannot possibly have so much personnel as to watch the actions of every individual person in the country.
The answer is to, effectively, "recruit" informants from among the people. In other words, have people watch other people, and report any sign of dissent or misbehavior. With enough propaganda and social engineering, a significant portion, perhaps even a majority, of people can be effectively "recruited" to become informants for the regime.
North Korea is a perfect example of this. From recent history, the former East Germany was, possibly, an even more perfect example of this.
This is exactly how East Germany succeeded in keeping their totalitarian regime for so long (about 40 years). People spying on other people, and having enormous amounts of informants, was everyday life in the country. Consider for example these paragraphs from the Wikipedia page about the Stasi:
"Full-time officers were posted to all major industrial plants (the extensiveness of any surveillance largely depended on how valuable a product was to the economy) and one tenant in every apartment building was designated as a watchdog reporting to an area representative of the Volkspolizei (Vopo). Spies reported every relative or friend who stayed the night at another's apartment. Tiny holes were drilled in apartment and hotel room walls through which Stasi agents filmed citizens with special video cameras. Schools, universities, and hospitals were extensively infiltrated."
"The Stasi infiltrated almost every aspect of GDR life. In the mid-1980s, a network of IMs began growing in both German states; by the time that East Germany collapsed in 1989, the Stasi employed 91,015 employees and 173,081 informants. About one out of every 63 East Germans collaborated with the Stasi. By at least one estimate, the Stasi maintained greater surveillance over its own people than any secret police force in history. The Stasi employed one full-time agent for every 166 East Germans. The ratios swelled when informers were factored in: counting part-time informers, the Stasi had one informer per 6.5 people."People ratting out other people to the authorities is one of the hallmarks of totalitarianism. Naturally the regressive left is using the exact same tactics. For example, consider this article: Williams College Students Can Report Each Other for 'Making Comments on Social Media' About Religion or Politics
"Williams College is one of at least 100 campuses with a system in place for students to report each other for saying or doing something slightly offensive. These trivially disturbing occurrences are known as "bias incidents"—and at Williams, virtually anything could qualify."The scary thing is that these are people who in the near future will become politicians, law-makers, judges, police officers, CEO's, and teachers.