An interrogation droid, also known as a torture droid or pain droid, was used as a torture device in coercive interrogations. Various varieties existed, equipped with various implements of torture. Sometimes, they were modified medical droids. While they were technically illegal, Darth Vader used one during the interrogation of Princess Leia Organa in 0 BBY.
The alien and mechanical quality of a droid could make it more frightening than an organic interrogator, which often aided the interrogation. A droid was also more capable of finely judging a subject's reactions to torture, and of applying various "treatments" precisely.
For the interrogator, using a droid was sometimes less troubling than performing the torture oneself. If the interrogator wished to assume a bargaining tone with the subject after the coercive phase, the use of a droid also helped redirect the subject's anger towards the droid, rather than the interrogator.
- Star Wars: Tiny Death Star