Holding governments to account for their abuses of human rights is hard. It requires focused forensic investigation, impartiality and no little courage. For six decades, Amnesty International has been advocating for prisoners of conscience in the face of state repression. But in recent times it’s both broadened its focus and faced unprecedented criticism. Stephen Sackur speaks to Agnes Callamard, Amnesty's secretary general. Is her organisation facing a credibility crisis?