My experience is that the green pit vipers are far from aggressive. I can't recall any event of one even trying to bite, even when handled with a snake hook . Surely if you really grab them, it will likely be a different story. I only use a hook, usually to move them off the road at night.
Pit vipers rarely flee when approached in daytime. Some individuals do try to move away if you really push the lens in their face. But seems like you won't try to get that close ;)
Common does not mean easily seen. It's the reason I love looking for snakes. I get most enjoyment from spotting animals. It's basically nothing different than the game hide-and-seek. The first second your eye spots it, gives a moment of joy and adrenaline. And the enjoyment is at its best when it is a well-camouflaged creature blending in their surroundings. Snakes are often well camouflaged and perfectly blending in, so that makes it extra special when you manage to spot them. Especially the pit vipers are always fun. They blend in so well, and because they don't move they are hard to see, at least in daytime. This season when there is more rain they often choose to wait in ambush sometimes just inches above the ground, head pointing down, waiting for frogs. But I have seen them up to 10 m high up in trees.
Surely you need to get habituated to know where to look, but even then, it can be very hard. They like humid conditions, generally rather dense undergrowth, and shaded areas. It's rare to see one in the sun, except in the winter months when I have seen them at the tips of branches hanging over the river, where they get sun through the little leaves above them and even sun from reflection on the water level. And they stay on their spot for weeks sometimes couple of months. In the rain season it's usually just a couple days that they stick to one spot and then move on.
At night they are a lot easier to see, because their bodies seem to really standout, almost fluorescent green, when you point a flashlight at them. A snake nearly invisible in daytime, can be hard to miss at night.
And then you learn how many you walked by in daytime without noticing ;)
But there is little to worry about snakes. The only snakes that worry me a bit are the Malayan Pit Viper and the Siamese Viper, they lay between leaf litter on the forest floor, blending in extremely well.
The good news is, that at least in the accessible part of Khao Yai you won't see these species.
If wearing proper boots, you don't have much to worry. My shoes are not so protective, but if I go to an area where I know these species are present I change to boots.
The other snakes like e.g. the Cobras are very fast, and will usually move away. They don't lie around waiting for you to step on them ;)
Anyway, next time you are in the park, keep an eye out for the vipers, they are beautiful !
Most of the time seen between eye-level and just inches off the ground.
Enjoy your time in the forest!