Multiple sclerosis is a hell of a lot like Randall in this scene. We can't see it - we never know when it is going to attack, or what part of our body will be caught in its evil grasp. Other 'invisible' illnesses are like that too - you just never know.
If you recall - also in this scene of the movie, Mike comes in and is talking with Sulley - and Mike can't see that Sulley is being attacked. It takes a bad throw of the snow-cone ball to make Randall visible to Mike & eventually help Sulley overcome his attacker.
This fights right into the people that are in the lives of the person living with the invisible illness. They can't see what is going on - and a lot of times they just don't get it - they don't understand that we can't always plan ahead - or drop things for a last minute party/gathering. People cannot see the fatigue we battle. People cannot see the weakness we endure. People cannot see the energy just getting dressed can consume. People just don't understand (easily) what they cannot see. Hell - I'm living it & I don't always understand it. What I wouldn't do for a friend to throw a ball of snowcone & make the enemy appear so I could finally grab hold and fight it back.
I write all this to say that you never know what someone else is dealing with - what just being where they are at that very moment might be costing them. If you know that someone is living with a disease/illness that you can't see - do all that you can to be a little more patient. Understand that sometimes, they just want someone there to hold their hand in silent support... and maybe throw a snowcone at them every now and then. :)
Snow cone anyone? I swear - its lemon!