I interviewed there last year. The whole WashU interview process hinges upon your 20 minute committee interview, and maybe a little on your follow-up meeting with one of the committee members. Do not worry so much about any of the other interviews. The research faculty interviews are just for you to get to know faculty with whom you might want to work. They really try hard to let you speak to all the people you request. The MD interview is basically a formality. Basically, if you get in to their MSTP, they will make sure you get into the med school. I have heard of instances where they flew people back to redo their MD interview in the rare cases where people got in MSTP and not MD. As you will see, the MSTP at WashU pulls the strings at that school; they really throw the weight of their huge endowment around.
Now as for the committee interview, you will sit at the head of a table in a small room with 10-15 committee members sitting all around. They will first ask you why you are pursuing the dual degree. This will probably only take a couple minutes. Then, they will ask you to tell them about your research. As you go along, they will interrupt you and ask you questions. A couple words of advice: first, do NOT make anything up! There will be a lot of experts in many different fields sitting around the table. It's likely that one of them will ask you a question for which you will not have a good answer. Though you do want to show your knowledge, it is MUCH better to say you don't know an answer than to state something false. They really are looking for maturity of thought. Second, be excited and enthusiastic about your research. Sell to them that your research was meaningful and helped you grow as a scientist and thinker. Since several committee members are likely to know little about your field, give them enough context so they can also get excited about your work. Lastly, try to get all the way through your story. Twenty minutes is a very short time, and they will be asking lots of questions; it will be largely up to you to tell them everything you want them to know. I remember talking really fast and steering my responses back toward my story. You will also get the chance in your follow-up meeting with a committee member to fill in any last details, but it's best to leave a strong impression on the committee as a whole. If you did well, you might even get some instant feedback in your follow-up meeting (say, spending the whole time convincing you to come!). The committee then meets again the following week to make an official decision whether to accept or reject (there is no waitlist at WashU). Then, you DON'T want to get a phone call from them (unless they've changes this). No call means you're app is on the way to the MD side to get approved. A call, however, means bad news...
This info may be a year old, but I hope it is helpful.
