We call them striped unicorns — that ideal expert for a given case, a respected, unimpeachable authority with exceptional communication skills and an unflappable demeanor. It can be difficult to differentiate the ideal from the not-quite-right when legal teams are under the gun to find an expert, but there is a methodology for ensuring their suitability.

As experts in trial strategy who routinely assist clients involved in high-profile, white-collar criminal cases, we are always struck by the influence the federal government has over the evolution of these cases. The rules of criminal procedure and the resources of the government seem to put even the most wealthy, well-represented defendants in an imbalanced, unfair predicament.

Last week, lawyers for Brendan Dassey, whose conviction was documented in the Netflix series “Making a Murderer,” asked the U.S. Supreme Court to review a federal appeals court decision that ruled a young Dassey’s confession was voluntary. He was 16 years old at the time he confessed to helping his uncle, Steven Avery, rape and murder photographer Teresa Halbach in 2005, and according to court filings, he has suffered from intellectual disabilities most of his life. Dassey’s attorneys claim investigators took advantage of his youth and mental deficiencies to...