I just want to delve into a little wormhole about how Egon always pops his collar as it is a small detail that has always interested me.
If we go back to when Egon isn't in the Ghostbusters uniform he wears a suit and tie, making him look like a stereotypical old-school scientist. He is never seen in a t-shirt or a shirt that is unbuttoned which is clear in a deleted scene in Ghostbusters 2 when he is the only Ghostbuster who does all of the buttons up on his shirt when they are committed to the psych ward.
The fact Egon never wears anything other than a blazer, shirt, tie, and trousers when not in the Ghostbusters uniform suggests that he is the most comfortable in these clothes, even if they are considered formal and only suitable for special occasions. This implies that Egon is uncomfortable in more comfy clothes like t-shirts as he has multiple opportunities to wear them but never does.
This means that the Ghostbusters uniform would be a big step outside of Egon's comfort zone as he doesn't wear anything different and if he does, like his pyjamas it still has a collar and buttons. Therefore, popping the collar may give Egon a sense of security and comfort as it reminds him of the collar on his shirts, which is a part of his clothes that makes him feel comfortable.
I also think it's clear that Egon doesn't like change as even his usual clothes are the same; he has the same dark grey blazer and trousers on in every scene and more often than not he wears a grey jumper under his blazer. The tie changes (though they are usually in the same colour pallet) and so does the jumper as it's sometimes a different shade of grey, but I think it's likely that the reason it changes is because his usual, light grey jumper was unavailable as it was being washed or something but the main thing is that it is still grey.
It's clear that Egon keeps to a pattern with his clothes and the Ghostbusters uniform completely breaks his pattern. Therefore, it makes sense that Egon would pop his collar as he would be keeping the pattern of having something around his neck e.g. his shirt's collar, but he would also retain a sense of regularity and comfort as he has a bit of his normal, comforting clothes with him even if he has to create it by popping his collar.
However, there are some instances when Egon's collar is not up but is actually down. Most of these occur because the collar has been forced down by an activity or a movement and he instantly puts it up when he gets the chance which is shown when they get let out of the psych ward in Ghostbusters 2 as in the background you can see (barely but it's there) that as soon as Egon has his uniform on and before he's zipped it up he goes straight to the collar and pulls it up. However, sometimes it is purposefully down and that mostly happens when he is with Ray.
It's clear that Egon and Ray have a great relationship and the strength of the relationship can be verified by the way Egon wears his clothes.
This picture has always fascinated me firstly because Egon has not popped his collar but also by the fact that the zip on Egon's uniform is undone. In every other scene the zip on Egon's uniform is close to the top, so close that you can barely see the black t-shirt underneath but with Ray his zip is completely undone and I think this shows that Egon is the most comfortable around Ray. When Egon is around Venkman or Winston he wears his uniform with a popped collar and a fully closed zip but with Ray he does not do this. When he is around Ray, Egon wears his clothes in a more lazy way, which is a complete contrast to Egon's usual character implying that Ray makes Egon feel relaxed and comforted as he drops some of the formalities within his clothing and doesn't pop his collar.
In conclusion, Egon hates change and has a set of clothes that he finds comforting and deviating from them causes him discomfort so he tries everything to make himself more comfortable, the main thing being popping his collar. The way Egon wears his clothes says so much about his character and the small touch by Harold Ramis of popping Egon's collar says a lot about Egon and his personality.
Sorry for the long post but I just needed to get this off my chest.
