What type of denial is it?
Your subject can offer a verbalised denial in one of two ways; emphatic or with explanation. There are some basic rules we follow when dealing with either form of denial coming from the guilty. Handling the explanatory denial properly could lead you to a short-cut to the truth.
An explanatory denial typically starts like this, "I would never do that because..." You should view the explanatory denials as very often being a true statement, which tend to come from the guilty. Tactically this type of denial is made by a guilty person in an attempt to sidetrack the interviewer from the issue at hand. When your subject gives you this type of denial it is very important that you accept it as if it is true and immediately spin the explanation into your rationalisation.
Your quick response to the explanation should begin with, "Thank you, so that tells me..." or "Great, so it must be..." or "Now I think I understand..." All of these responses allow you, the interviewer, to avoid arguing with the subject, refocus their attention and maintain control of the interview.
Please remember that all denials have to be evaluated within the totality of circumstances as to their level of potential truthfulness.