To prevent punishing the innocent they should be offered to testify under the influence of sodium amatol. I can tell you that it works. I did it to save my life in 1977 in Austin, Texas after being accused of indecency with a child and threatening to kill the victim. The psychiatrist that my lawyer sent me to for a psychological profile got a law degree while he was in med school so he ws pretty smart. We did the truth serum interview in a hospital and I was totally out of it for eight hours after we left the hospital. The psychiatrist went to the grand jury to testify but the assistant DA talked to him in the hall for a minute and asked him to wait - costing me $200/hour . The punk AD went in a told the GJ that I was innocent and the case ended without the SA interview being heard.
In the case of a capital crime if the convicted person wants to do the SA testimony and confesses he could just be given another shot of pure heroin. If he says that he didn't do the crime he could wake up at home in his own bed.
I just called the Dr. to thank him again for helping me and he told me that he had done a SA interview on a guy who had raped a girl, cut her throat and bumped her in a land fill but she lived and told who he was. The Dr. told the court at the punishment phase of the trial that the guy was the most dangerous person he had ever met but they gave the guy 10 years and when he got out he killed several women before he was caught.