I have never heard of an assessor appearing at a tribunal, the system is essentially setup to protect the assessors from accountability.
DWP turning up is very rare also, maybe a higher chance at upper tribunal. This is probably a good thing, as been not represented should improve a claimants chances.
At a tribunal I think its just best to get rid of emotions. Concentrate on what you need to do to change the decision, on my DLA tribunal my effort was put into discrediting the assessor, which in turn led to them completely disregarding the report. Make sure you make eye contact when speaking to them, address them properly, and be honest and cooperative. If they find you credible then your evidence (even without medical supporting evidence) will bear weight. Make sure you know where you stand in law before going there.
The doctor's at tribunals tend to concentrate on questions that are actually relevant, but they cannot physically examine you. You may get grilled, but understand they have a job to come to a decision in a very limited amount of time.
The tribunal are on no one's side but the law.