I find I keep writing the same note to my students (or explaining it to them) so here goes.
How to prepare for the defence:
1. In the week before, reread your thesis. Make notes on any corrections you know you need to make in advance of the deposit, and any areas where you a) have changed your mind b) need to clarify something c) wished you’d been able to expand on what you’d said. Remember that most people don’t really get a sense of the argument or importance of their thesis until quite late in the game, so you may well see things you didn’t see before. Of course, most theses are completed under time pressure so you will notice imperfections. That’s normal.
2. Write a short introductory statement. It can include audio, video, images, but it can also just be talk. For more on content, see below.
3. Practice the statement. Time it. You don’t want to go too long. Or to think too hard about it in the moment.
4. The day before the defence, take it easy but have stuff to do if you are prone to getting nervous. Get a good night’s sleep the night before the defence–by any means necessary.
What happens in the defence:
The dissertation defence is one of the most ritualistic of academic rituals. It is also weird. It is the one time you are in a room with a bunch of smart people who are ready to give you feedback upon having read your work, and yet the ritual makes that difficult. Every school does it differently, but at McGill, it works like this: There is a committee of six people, one of whom is a “pro-dean” who runs the defence and rarely asks any questions. The rest of the committee consists of the supervisor, one person from outside the department (they can be from another school), and three other faculty members, one of whom is the internal examiner. Since McGill doesn’t pay for external examiners to come to the defence, the supervisor usually reads the external examiner’s questions, because, let’s face it, you’ve heard plenty from your supervisor about your thesis at this point. Defences are public, so you can invite your friends, family, etc. Some people like to pack the room for support and some tell everyone to stay away.
The defence opens with a statement from the candidate of 15-30 minutes (it can be shorter, longer will extend the duration of the defence, which can be a problem). The official purpose of the statement is to give an overview of the thesis, expand on material you want to expand upon, clarify things, show examples, retract things or change your mind, and so forth. The actual purpose of the statement is just to get your warmed up.
The number one problem with doctoral defences is that the event is usually unlike anything the candidates have done before, so they are nervous. This makes it harder for candidates to hear what’s being asked of them and to think and be eloquent on their feet. So the warmup is super important so when you get that first question, you’ve already been thinking and talking, and setting the tone.
Once the intro is done, questions go around the table, from the most “outside” committee member to the supervisor. This is usually done in a round of 15 minutes per person, and then a shorter second round. Here is where we get to the number two problem. Professors don’t always ask good questions. Or they may not even ask questions. So your challenge, while you’re nervous, is to hear what the person is asking. Bring a pen and paper and take notes. Sit and think a minute after they get done (the opening statements of committee members can sometimes go on as they begin with a reaction to the thesis). Did you understand the question? If not, ask for clarification. What do you want to say? What the most important part? THEN, give an answer starting with the most important point first.
Questions will go around the table. A second round might be more interactive, or not. Then, the pro-dean will invite questions from the audience. This is the point at which your friends should realize they are prolonging the defence by asking you questions.
After all the questions are exhausted, everyone leaves the room except for the committee (or the committee deliberates somewhere else). Then you get invited back in.
A thesis can pass as-is (almost never), pass with minor revisions that must be approved by the supervisor but not the committee (most common), pass with major revisions that must be reviewed by the committee (sometimes), or fail (extremely rare). I have been on something like 60 doctoral defences at this point and no thesis has outright failed at the point of the defence.