I'm an official fence judge tomorrow (well, an official assistant fence judge), and my fence is #23, the Keeper's Brush. It's a pretty straightforward fence, though it's high (over 4 feet) and wide (over a 9 feet spread!). I'm pretty darn excited; every one of these great riders and horses will have to come by me. I'll be on course at my fence by 9:15.
As a fence judge, I have a pretty large responsibility, which includes (but is not limited to): making sure the jump remains clean, safe, and no observers interfere with a horse/rider's approach/taking the jump. I need to assess if the horse refuses, runs out, or if the rider falls. If a horse or rider falls, I can help the rider, catch the horse, assist the rider if he/she would like to re-mount, etc. I need to stop the official time if this happens. If it takes a long time, I'll need to stop this rider and allow the next rider to pass before letting this rider continue.
There are several helpers at each jump. Even a straight forward jump like mine has a senior fence judge, an assistant fence judge, a communications person (who communicates scores, etc.), and another official scorer. The more complex jumps have even more people to help.