A well-conducted interrogation is not torture, which in practice is widely known to be ineffective at producing reliable information. Prisoners of war routinely undergo interrogation, which the laws of war permit.
A typical interrogation involves three people:
The interviewer usually asks short open-ended questions and attempts to establish a friendly rapport with the interviewee, and speaks with a neutral and sympathetic voice. The notetaker watches the interaction between the interviewer and the interviewee, paying particular attention to behavior and body language.
One notable interrogation technique is the Reid Technique.
Note that no prohibition forbids the interrogator from lying, from making misleading statements or from implying that the interviewee has already been implicated in the crime by someone else. Deception forms an important part of effective interrogation.
However, threats which the interrogator cannot actually carry out are generally ineffective unless the subject is young, inexperienced or terrified.
Compare good cop/bad cop.