If Agents can take over anyone plugged into The Matrix (until they swallow the red pill), then when other redpills approaches the normies to free them, why don't the Agents just take them over?
The Agents do not appear to be omniscient within the world of the Matrix, relying on detecting anomalies, then traveling bodily to the location of the anomaly in order to deal with them. It only appears to be in extremis (or hot pursuit) that they use their ability to jump into bluepills, even when tracking high priority targets like Trinity.
Their ability to detect redpill activity is also likely impeded by being outside the "core network" mentioned by Morpheus in Reloaded. This combination of factors seems to give Morpheus' crew sufficient confidence to take an unbugged bluepill to their location but not without first ensuring that they have plenty of guns pointed in his general direction until they're pretty confident that their remoteness means that he isn't likely to be taken over.
Morpheus: Get us out of here, Link.
Link: That won’t be easy, sir.
Morpheus: I know. We’re inside the core network.
Scanner scene from Animatrix: Beyond
The agents are an allegory for the way ordinary people who believe in the system can act as agents of the system when they see somebody act in a way contrary to what they have been told is good and right. Remember, Trinity and Morpheus are known as terrorists. What would we do, on spotting a known terrorist or someone acting in a suspicious way in the street? Why, we would be a good citizen, and act as an agent of the consensus social order in trying to bring the enemy of society to justice.
'The Matrix' is the set of norms, assumptions, rules we follow "fitting in" to society. JS Mill spoke of needing to guard against "the tyranny of the prevailing opinion and feeling; against the tendency of society to impose, by other means than civil penalties, its own ideas and practices as rules of conduct on those who dissent from them; to fetter the development, and, if possible, prevent the formation, of any individuality not in harmony with its ways, and compel all characters to fashion themselves upon the model of its own." One of the many meanings of the word 'Matrix' is "the cultural, social, or political environment in which something develops". It's also a word for the mould in which a metal shape is cast. It's an old word for the womb, the space in which a human is formed, and into which we fit. And it's a word used in post-modern philosophy (e.g. Baudrillard's Simulacra and Simulation, which appears in the film) to refer to the way society moulds the character of people living in it, defining what is acceptable behaviour, and triggering violent anger at people who break the unwritten rules.
So consider an example no doubt close to the hearts of the Wachowski sisters - that of sex roles as enforced in the past. There are 'rules' on what boys are and are not allowed to do, and other rules on what girls are and are not allowed to do. And if a boy was caught doing something only girls were supposed to do, it's not just the police or government you have to worry about - any random person could take it into their heads to act violently to enforce society's rules. Your family, your friends and work colleagues, or just some random thugs on a bus. This is what being an Agent is about.
It's not just sex roles, of course. In the past there were moral rules on numerous topics, like segregated racial roles, sexuality, sexual behaviour, and religious orthodoxy. Today, we enforce a different set of moral rules against racism, sexism, scepticism of authority, and 'political incorrectness' just as fiercely. It's a different version of the Matrix, but just as confining, and just as intolerable and totalitarian to those who don't fit with the rules of the day. For it to work as a 'social contract' requires that we are given a choice, and are genuinely free to opt out. This is what the utopian 'Architects' of society never understand.
Society imposes rules on us in order to allow the peaceful coexistence and cooperation that powers the economy and technological civilisation (like a battery!). We fit into our fabric-covered boxes in offices like cogs in a machine. But it doesn't work without Freedom. Slaves given no choice reject the programming, even if that would result in a breakdown of the system that keeps us alive. But if given the choice, most of us do choose to fit in, to work for the greater good of society. But the greater good of society is not justification for taking the choice away, and trying to force conformity with social norms ultimately leads to rebellion and revolution, and the creation of a new consensus. The Matrix has to be chosen, it can't (sustainably) be imposed.
So from this point of view, we can see that rebels against society's rules would be immune to being taken over. They don't believe in the rules. That's why the Agents had to use other means to track Neo when he refused to cooperate with them. It's only true to say that 'anyone' can be an Agent in the sense that you can't tell just by looking at someone on the street whether they are a Believer who will violently enforce the rules. But as with all things in the Matrix it is subject to choice - only those who choose to will do so.
The unplugged only approach people who they believe are rebelling, and willing to fight for Freedom. Agents can't take them over, but they can cut them off from society. Being 'unplugged' means being able to survive as a 'heretic' outside that social system, without other people's support or approval, with every hand turned against you. The Matrix is a deep philosophical allegory for the way human society works. If you understand the allegory, it answers many other questions too.