An informant is someone who is part of or has infiltrated an organization where criminal activity is occurring. They're paired with a contact within the police force, providing inside information that those on the outside wouldn't be able to access. In exchange, they may receive immunity for their own offenses, a commuted sentence, or monetary reimbursement. This relationship is a secret one that most people won't be aware of; this, commonly combined with an imbalance of power in the relationship, can generate high stakes and a lot of conflict.

Each relationship is different, depending on the people involved, their history together, their individual personalities, and a host of factors. Here are some common dynamics—healthy, and not so much—that can accompany this relationship. Use the ideas that suit your story and work best for your characters to bring about and/or resolve the necessary conflict.

Each party recognizing what the person brings to the table
Both parties respecting the agreement or arrangement (maintaining an informant's confidentiality, not shopping around information to other agencies, etc.)
Both people respecting the other's boundaries (the officer not showing up at the informant's home or workplace, the informant not expecting the officer to drop everything and come running when he calls, etc.)
Each person approaching the relationship as equals rather than one person having unlimited power over the other
The officer validating the informant's feelings rather than dismissing them
The relationship becoming too cozy or friendly
The informant acting as if the police work for him
The officer threatening or browbeating the informant
Either party not trusting the other (because the officer didn't responding when the informant requests help, the informant provided false information, etc.)
A paid informant shaking down the police for a better deal in the middle of an investigation
The informant calling at all hours, on weekends, on holidays, etc.
An informant crossing lines that threaten the officer's case (trying to entrap a suspect, etc.)

The informant's cover being blown
People within the organization suspecting a mole and becoming more tight-lipped and careful, making it harder for the informant to get information
Budget cuts within the police agency that threaten the informant's pay
A change-up in the organization that removes the informant's access to the right people or materials
The officer who works with the informant being wounded or fired
The police officer uncovering information that makes the informant look suspicious, as if he may have been compromised
The informant being arrested and no one knowing they're working on the inside