To add to the excellent ideas presented thus far, I would suggest considering the possibility that some prospective participants may have lost hope, having seen the outcomes of prior trials. I would not be contributing this idea if I agreed with that point of view; however, I think it may be important to acknowledge that disappointments have occurred, and provide an explanation of why it is believed that future trials may have some reasonable probability of success.
As an example, consider the aducanumab trials. Prospective participants may remember the past disappointments with medications that target amyloid, and question why aducanumab should be any different, especially if they have already started to experience clinically-detectable symptoms. Researchers should be prepared to answer these questions during recruitment, and perhaps even prior to this stage, when advertising the trial and communicating with the media. Scientific details may be provided to those who are interested. The idea is to provide people with a thorough explanation of why their commitment is worthwhile, and to avoid asking for blind faith in the value of the research.