Most judges will question you directly at the hearing, though some judges prefer that the representative ask all the questions. At the hearing, your representative can ask whatever questions he thinks important. Generally you testify as to your history and your symptomatology. The doctors usually supply evidence in the form of written reports.
Certain cases are definitely easier to win than others. If you are statutorily blind, you are automatically disabled. If you have ALS, it is likely that you will be awarded benefits right away. Psychiatric cases are generally harder to win than others because many psychiatric patients fight the process or they don't see their doctors regularly or stay in treatment. It is very hard to win a pain case where someone alleges a great deal of time and there is no readily determinable injury. For that reason, people who have fibromyalgia and chronic fatigue disorders have trouble winning cases.
The older you are, the more you are felt to not be trainable and less able to adjust to new work environments. Therefore, you are less employable. It is always easier to prove eligibility for Social Security Disability if you are older than if you are younger.