Psychiatrists often do evaluations. It is where they get basic information, and how they decide what will be done in a given case. Ordinarily a psychiatrist is involved in helping a patient in some way. But in forensic psychiatric evaluations, the relationship is much different. This would be a challenge for professionals in this field, and specialized training is needed for the different approach required.
A relationship of some kind forms, an alliance of sorts, between the psychiatrist and his or her patient. The patient trusts the one doing the evaluation and has no reason to withhold any information. The information is also kept confidential in almost all cases.
A forensic psychiatric evaluation however, creates a much different relationship between the examiner and patient, and the goals are different. Many of the same tools and observation techniques, and even questions, may be the same in both cases, The professional doing the evaluation must shift gears to do forensic work in a big way.
Forensics often implies police work, and it could involve that. More often though, it involves cases in court and the evaluation is part of the process of preparing a case.
The forensic psychiatric evaluation may determine if someone is competent to stand trial, for instance. Does the person understand enough to be declared competent. And if a person is declared competent, they may then call upon a psychiatrist weigh on on whether the person could be declared innocent by reason of insanity. That often results in a lesser charge if so. In both instances, the examiner may be on one side or the other, but is not supposed to be biased toward the patient.
Another aspect is that in these cases the person may be motivated to lie, so the worker has to be able to spot dishonesty as well.
Professionals in this field can also work for attorneys on either side for things like witness reliability, jury selection, and other things that are out of the public eye. Sometimes they also work with crime victims to determine if the victim is telling the truth, or is in some way being less than totally honest.