Very casual, but don't show up dressed casually! The interview was mostly behavioral questions, all necessary training can be completed on the job. LDS Philanthropies is affialiated with the Church of Jesus Christ of Latter-day Saints, so you should expect the situation to reflect those values. It was really important for me to have researched LDS Philanthropies and to understand what their purpose is. That also helped me enjoy the work by helping me understand what the mission I contributed to.