In garrison, your job is identical to the work a regular doctor does. Sometimes you get posted in a hospital, and sometimes you get stuck doing clinic hours for whiny trainees. It's dictated mostly by your specialty.
Interestingly, because garrisons include a large number of dependents and old vets, your patients aren't demographically different from a regular hospital. I'd suppose there'd be a larger portion of sports-type injuries, tho...
In the field, you do the same thing, but you work out of a tent and see a bit more trauma patients.
