This chapter elaborates the psychophysiological basis of a forensic assessment that helps in eliciting truth from a person during a forensic assessment interview (FAINT). The FAINT is set up as a scientific experiment where the only stimulus presented is the interviewer's question, and all extraneous stimuli are controlled. A FAINT utilizes relevant questions dealing with the crime, to pose the greatest threat to the guilty suspect because he will be forced to either confess to or lie about the matter at hand. Comparison questions designed to deal with earlier transgressions or peccadilloes are utilized to threaten the innocent suspect. Under these circumstances, when a suspect lies, emotional changes occur because of conditioning, conflict, or psychological set. This emotional imbalance causes subsequent physiological changes resulting in observable behaviors, the degree of which may be affected by various factors. These factors include the interviewee's perception of the interviewer's ability to detect deception, the interviewee's past experiences at deception, and the interviewee's perception of the seriousness of being caught. Through the use of relevant and comparison questions, and given the ability to observe and detect changes associated with sympathetic arousal, the trained interviewer can monitor the suspect's psychological set and solve the puzzle of truth or deception.