My fear would be fixating on him or the bike and then running right into/over him/it.
It's taken several years to get used to it.
When somebody crashes in front of me now, I actually have trained myself to YELL in my helmet "Don't look at him, don't look at him, don't look at him" ... seriously, that's how I do it.
I've even run over parts of other people's bikes ... luckily no actual person. There's not much you can do if it happens close in front of you. The bike wants to keep moving a straight line and it's so hard to change direction in half-a-second.
My three biggest fears on the track are: 1) getting hit from behind; 2) running over somebody that goes down when "close racing"; and 3) a triple-digit high-side
The fact that I even have fears is why I could never be a comptitive racer.
LOL.