Once you get an interview invite, you've defeated 70% of the battle, and I think interviews fill in the remaining 30%. Your interview experience is highly dependent on your interviewer. I think the few people who don't get accepted post-interview are the ones who got unlucky and got interviewers who would continuously grill them.
I think interviewers are looking for 3 things from you: knowledge of your own research project (based on your ability to answer the questions they ask you about your research), ability to communicate your research project, and enthusiasm/ability to ask good questions about their research. I've attended 5 interviews so far, and I think all my interviewers try their best to gauge these 3 variables.