An expert consultant can assist with witness preparation, cross-examination, legal strategy, and review of medical records and of other experts' work.
Psychologists are known to consult in jury selection, which is a critical phase in the legal process. Unfortunately, there is little evidence that the assistance of a forensic consultant in voir dire increases the likelihood of a favorable outcome. In addition, most cases will be resolved by a plea bargain agreement before ever reaching the jury selection phase.
This is not to suggest the use of a psychologist should be limited to evaluations and expert witness testimony. There is considerable evidence that forensic consultant are effective at the the tasks for which they are specifically trained, for example, assessment and treatment of mental disorders, critique of a behavioral modification program, interpreting the results of empirical research, describing the effects of metnal illness or mental disability, etc.
Psychologists who do not need to maintain the role of an objective expert can provide guidance and assist with legal strategy in the early stages of a case. A consultant might screen cases for merit or assist with obtaining and interpreting and medical records and behavioral data. A forensic consultant might also advise an attorney on how specific mental illnesses can affect their clients and how the legal system tends to respond to them, assist with interviewing clients and witnesses, gathering mental health related information, or preparing witnesses for testimony. In some cases, it may be more beneficial to have a forensic consultant review the work of an opposing expert and assist with cross-examination than have another evaluation done.