REC members often have the sort of experience which will be useful in scrutinising the ethical aspects of a research proposal. For example, they might be patients, members of the public, nurses, GPs, hospital doctors, statisticians, pharmacists and academics, as well as people with specific ethical expertise gained through a legal, philosophical or theological background.
REC members receive special training in ethical review and have the chance to debate challenging issues. They give freely of their time and expertise (although they are reimbursed for their expenses).
You can read more about becoming a REC member or participating in research, in the Patients and the public section.