This scenario is in its most embryonic stage so, if interested, feel free to suggest any sort of alternatives and embellishments.
The narrative develops entirely through correspondence between a male convict (let's call him Ronnie for now) and his female attorney (let's call her Abby for now).
Ronnie is at the beginning of a lengthy sentence, having recently been convicted of the armed robbery of a liquor store and aggravated assault of the sales clerk. In his early twenties, he has been incarcerated before, but only in juvenile detention facilities and, later, in county jails on short sentences arising from petty crimes. This is his first felony conviction and the first time he has been sentenced to a state penitentiary. He may or may not have committed the crimes of which he has been convicted, the primary evidence against him consisting of shaky eyewitness identifications made under what were perhaps improperly suggestive circumstances.
Whether innocent or guilty, though, Ronnie is unquestionably a sociopath. He is also manipulative, narcissistic, self-pitying, and adept at concealing these traits. He's no dummy.
Abby is a couple of years out of a very good law school, and has eschewed the opportunities offered her to enter big firm civil practice for a job as an appellate attorney in a public defender's office. She is a true believer in justice and defending the "oppressed". Abby is engaged to another lawyer (a corporate type) who is as mundane as he is self-absorbed and controlling. Sex with him has long since ceased to be much more than a chore.
Abby is assigned to prosecute Ronnie's appeal from his conviction. The two begin to exchange letters, initially limited to the facts and legal issues involved in Ronnie's case. Fairly quickly, though, Ronnie perceives the opportunity, through their correspondence, to play upon Abby's sympathies, to work his way into her affections, and ultimately to control her. In his letters, Ronnie reveals increasingly intimate details about his upbringing, the mistakes he now claims to regret, and the horrors of prison life. Some of these details are likely fictive.
Though Abby initially resists Ronnie's attempts to personalize their epistolary relationship, she is gradually but inevitably and completely drawn in. She begins to respond in kind, confessing increasingly personal details of her own life. The disclosures become more and more intimate, both psychologically and sexually. Abby suspects, and Ronnie eventually confirms, her disclosures are providing fodder for his masturbatory fantasies. Abby is, in turn, highly aroused by this and Ronnie becomes a regular actor in her own fantasies, both during and when discharging the obligations she still believes she owes to her fiancee.
This is about as far as I've gotten with my ruminations on this subject.
If interested, please PM to discuss further narrative and character developments.