Interrogator is a rank in the ranks of the Imperial Inquisition, and is usually the final step to becoming a full Inquisitor. As their name suggests, they are often experts at interrogating suspects, although at this stage in their careers they are also expected to develop a wide range of complementary skills.
For this reason Interrogators are usually paired with an appropriate mentor such as a full Inquisitor. An example of this was the initial relationship between Inquisitor Gregor Eisenhorn and Interrogator Gideon Ravenor. Only once they have proven themselves in a variety of situations may an interrogator be elevated to the rank of full Inquisitor themselves.
Sources
Eisenhorn Trilogy - novels by Dan Abnett