If only everything is the same when we are shooting in the field!
Whether a hasty rest against a tree standing, a log when kneeling or shooting out of vehicle, you need to make the most of the position you have. Simply getting a good sight picture and putting the butt somewhere on your shoulder can be the best you have. Getting a good shot away in these circumstances is what makes good field shooters and hunters and separates them from the "on the bench" shooters.
On the weekend I had to shoot some animals in culverts where the pipe was only 300mm high but the entrance was partly covered in dirt. The ground was uneven and had rocks and thick vegetation at the entrance. Just actually getting a sight picture was enough and I was trying laying sideways, upside down and all sorts of things to even put the scope on the animals! Then actually being able to have the scope align without hitting the top of the culvert and running out of room, having the torch at the right angle to see and contend with a less than agile body of an aging shooter
(jeez I was sore on Monday)
As JV said, shooting out of a vehicle has its own challenges, you may be leaning way out with the side of the mirror as a rest and your bum halfway out the seat, the animal moves, your driving buddy doesn't think of the best position for you, or you don't have the experience to set yourself up.
You just need to get the rifle somewhere on your shoulder sometimes and be able to get a steady but good sight picture! Obviously if its a hard kicker that will be even more challenging but in answer to your question, you need to be able to adapt. Mind you, I have more than once shot animals from the vehicle that ae well behind and not had the rifle even near my shoulder which is fun when shooting out of the wrong eye!