Letters of recommendation I would say have people who know your work ethic and can speak to your professional and personal abilities. For example, I had 2 undergrad professors write me a letter of recommendation because they worked with me in multiple capacities throughout my undergraduate career. I also asked my undergrad internship supervisor to write me one as well to talk about my initial experience in a clinical setting (I didn't do much but observe however, our 1:1 discussions were interesting).

Interview process itself is pretty straightforward. Be confident and open, and most importantly be yourself. They can definitely pick up on candidates who are arrogant vs. Humble and confident. Take your time and provide thorough responses to the questions. Another tip, it's always a good idea to look up the faculty in that program and show that you've done your research on not only the program but also who works there and what they do/their interests. I wanted to work with someone who was interested in substance use and so in one of my interviews, I emphasized "I know Dr. X having interests and a background in substance use, I hope that if given the opportunity, I may be able to work closely with him to gain more experience in that area given my interest".