One of the issues the professional interviewer needs to address during many interviews or interrogations is overcoming the resistance of the individual to tell the truth. The resistance may be there for many reasons, some related to deception or fear, and some may be related to other factors, such as the environment of the interview or the relationship between the interviewer and interviewee.
If you can identify why the resistance is there, such as a lack of rapport or trust between you and the individual, then you can work to establish rapport and trust. If it is environmental, such as conducting an interview in an area where other people can overhear the conversation, then you can address that and move to a more private area.
If the individual appears resistant and you believe they are withholding information, you can try to let THEM convince themselves to tell the truth. Ask them questions like, "Why would it be important for someone involved in a situation like this to tell the truth?" Don't let them escape with an 'I-don't-know', try to have them come up with some reasons, they might be very compelling and help you tap into their thoughts and concerns about the situation, and provide a path on how to overcome their resistance. Then you can ask them "What would the positive outcomes be for someone who told the truth?" Again, have them think about it and respond. This may help them find the reasons for them to be truthful. You can then ask "What would the next steps be for a person in this situation?"
This type of questioning taps into their thoughts and motivations on why they are resistant and can lead to a positive change in their mindset. It helps them think through the situation a little better, because they may be caught up in the fear of getting in trouble for something they did but have not taken the time to think of the positive benefits of telling the truth.
The benefits they come up with and verbalize may be as simple as "they will feel better about themselves after telling the truth", "they will get it behind them", "they may be given a second chance" etc. but it is coming from them, not us. If the interviewer tell the person statements like that it will not have the same weight to persuade and motivate as if the individual says the same thing.
A detective I worked with shared a story with me. He stated that another detective was talking with a suspect on a major case, but he was not being cooperative and the detective was not getting through the barrier, or resistance, he was putting up. He took a break from the interview and told the other detective the difficulty he was having and asked if he would try talking with him. The detective went in the room, made good eye contact with the gentleman and sincerely asked, "What's on your mind...what are you concerned about?" The detective said the guy started crying and said his girlfriend was pregnant and he didn't want to go to jail and never see his baby!
In the "old days" overcoming a subject's resistance was dealt with quite differently...rubber hoses, bright lights and phone books come to mind from many old-time cop shows! Contemporary law enforcement and professional interviewers don't have to resort to unethical, illegal and ineffective techniques...they only need proper training, a proper mind-set, and a determination to find the truth.