One of the vampire abilities that Nick Knight uses most frequently is the power of hypnosis (sometimes colloquially known as "whammying" someone). This is not only useful for getting confessions from suspects, but can be used to help witnesses recall things that they have forgotten or are too traumatized to bear to think about.
Visually, one knows that a vampire is hypnotizing a person when they stare at them intently, sometimes with the eyes going the characteristic vampire green-gold.
Victims of vampire hypnosis in the Forever Knight universe do not behave the same way as hypnotized people in other science fiction or fantasy series. For example, in the Star Wars universe, which employs a type of hypnosis frequently called a "Jedi mind trick," victims typically adopt a casual tone of voice and then agree to whatever was being told to them without being able to think for themselves. Moments later, they might be puzzled about their new opinion, but would not feel obliged to change it back.
Victims of vampire hypnosis, however, respond with slackening face muscles; and their eyes lose focus, or may even be shut. Typically, they typically repeat in a rote, almost monotone voice whatever the vampire has said while inducing the hypnosis. Essentially, people hypnotized by a vampire are put into a trance.
Not all human beings are susceptible to hypnosis. Those who are resistors do not show any outward signs; so there is no way for a vampire to tell in advance whether they will be able to influence a person or not. The fact that the majority of mortals can be hypnotized tends to lead to the assumption that any mortal can be dealt with in this way—a complacency that only leads to trouble when one of the rare resistors is encountered.
The ability of a vampire to hypnotize mortals was integral to Forever Knight from the beginning of the series. At the very beginning of "Dark Knight", the series pilot, Nick uses hypnosis on a persistent reporter.