Still thinking about fieldwork, I’ve cited an extreme instance of “participant observation” in the work of Germaine Tillion in Ravensbrück concentration camp, as well as the ethical quandaries of Sudhir Venkatesh in Gang leader for a day.
Among all the literature on the psychology of evil, as a chilling instance of fieldwork interviewing, Gitta Sereny held lengthy talks with Franz Stangl over three weeks in 1971, after he was belatedly sentenced to life imprisonment for co-responsibility in the murder of 900,000 people at the Sobibór and Treblinka extermination camps.  Stangl died of heart failure nineteen hours after Sereny’s last visit.
As she observes (The German trauma, pp.88–9),
I decided to find one perpetrator if possible less primitive and with at least a semblance of moral awareness, who, if approached not as a monster but as a human being, might be able to explain his own catastrophic moral failure. […] I found Stangl more complex, more open, serious and even sad than any of the others I had observed; the only man with such a horrific record who appeared to manifest a semblance of conscience.
Within the extraordinary circumstances of these prison interviews, Sereny’s account is a model of detailed and sensitive work, reflection, and even empathy; as she carefully elicits as much candour as can be expected, she meticulously documents both his self-delusion and her own feelings.
Also importantly, her interviews are founded on much research and personal authority—experience of the period in question, in-depth study of the material, and discussions with people who worked under Stangl, witnesses, and survivors. All such background is important for fieldworkers, enabling them to ask apposite and probing questions sensitively.
Equally sobering is Sereny’s discussion of her meetings with Albert Speer—which indeed he initiated after reading her book about Stangl.  She unpacks the different and complex feelings of the generations since the war, notably in “Children of the Reich” (including their early worries about right-wing sentiments in the east after reunification).
I can’t think of a close parallel for other societies lastingly afflicted by the ghosts of unspeakable evils—certainly not for China, where what little we know of the “catastrophic moral failure” under Maoism (which was very far from being limited to the “usual suspect” of the Cultural Revolution) tends to be through the accounts of with victims rather than perpetrators—and it’s just as important to explore the mass of cases along the spectrum between those extremes. And of course, whereas such exposés depend on free media, for China such first-person reflections might only ever be muffled at best—since despite a change of direction, the same Party remains in power. Anyway, Gitta Sereny’s interviews are a model.
 “Colloquy with a conscience”, in her brilliant book The German trauma; more detailed is her book Into that darkness.
 The German trauma, pp.266–85, based on another of her books, Albert Speer: his battle with truth.