Lets assume that you interrogate a person that is in custody that has nothing to gain to help you:
+ Your opponent is in custody by you.
+ Interrogator specialty
- Your opponent has nothing to gain by helping you.
One + and one - will cancel each other out, so thanks to your interrogator specialty, you will still have an advantage when interrogating this person.
It is true that the specialty doesn't help you if it was already easy to interrogate in the first place, like if there was no disadvantage to the roll.
“Thanks for noticin' me.” - Eeyore