The doctoral candidate is responsible for choosing the members to serve on the dissertation committee. The dissertation committee supervises and advises the student through the process of writing his dissertation, as well as being the jury when he presents it orally. It is in the student's best interests to start getting to know professors early to learn their personalities, area of expertise and which ones would be the best fit for working together. He has to attend committee meetings and respect the time of the members. One of those members will be the student's primary sponsor.
Many doctoral programs require students to spend a semester teaching in the field before their doctorate can be granted. The teaching may be with the direct supervision of a Ph.D., assisting a teacher with a doctorate, or indirect supervision by a department head with periodic reviews based on student knowledge and class sit-ins.
Qualifying Exams are generally exams that test the student on substantive issues in her field of study. She is expected to be able to apply the concepts she has learned through the study of the field. There may be written and oral components to these exams. There may be a limit to the number of times a student can take the test and a set score she'd need to meet in order to pass.
The student is solely responsible for defending the position stated in his dissertation. His sponsor or dissertation committee cannot help him. They may even be his judges. This is generally a face-to-face meeting with the dissertation committee and possibly an additional review board. He may be asked oral questions to defend his position as well as receive some notes that will be addressed in a written response.