An interesting resource, regarding this topic, is the US Army's Vietnam Era interrogation field manual (published in 1969). It's available for download in two parts:

FM 30-15 Intelligence Interrogation

FM 30-15 Part II

In the second link, Chapter 4, Interrogation Support for Stability Operations is of particular interest - our current FM 2-22.3 HUMINT Collector Operations does not go into this subject in the same degree of detail. In light of the topic of this thread, I'd like to quote a bit from Section 4-7, Insurgent Vulnerability to Interrogation:
Humane treatment of insurgent captives should extend far beyond compliance with Article 3, if for no other reason than to render them more susceptible to interrogation. The insurgent is trained to expect brutal treatment upon capture. If, contrary to what he has been led to believe, this mistreatment is not forthcoming, he is apt to become psychologically softened for interrogation. Furthermore, brutality by either capturing troops or friendly interrogators will reduce defections and serve as grist for the insurgent's propaganda mill.