We don’t have the details, but we were given a vague explanation of it being a battle of wills of sort, where the interrogator has to force the criminal to show a memory, while the criminal can try to some something useless instead. While it was not mentioned, I would suppose that having more mana would give one an advantage, whether you are in the interrogating or defending position.

That it’s possible for the interrogator to guide the memory search to some degree at least is also clear from the fact that it’s possible to “interrogate” corpses, or just detached heads, albeit we were told that the memories from corpses are difficult to access and most often fragmented.