-cool one i just remembered was that you can email hunting pictures to anyone on your contact list-
and almost every single one of them will have unique replies to the dead bodies you show them lol
-if you take the flying lessons before heist 2 offshore but are still slow, T criticizes you for not taking enough
-the obey rocoto (audi q5/porsche cayenne) that gets stolen by the airport is is super customized... and if you keep it, the women still sends you a thanking email and a free suit ;/
-i dunno if it's unique but after you steal the cargobob for heist 2, there's a Sandking waiting for you at the hangar and it has a set of bullbars without the lower face body kit.... i dunno if that's unique but i never see sandking's with default bullbars and to put bullbars on at LSC, you have to also add that huge lower face body kit which means the truck looks super blocky.
-a lot of missions have different scenes depending on who you start with... i think we all know the heists do, like heist 2 is either a shoulder rub or a leg rub... heist 5 is either M threatening L or L and M making a firebomb with gas and toaster ovens... and other missions, if you show up with a character, you may miss half the scene like if you start Hood Safari with T or the Bring House Down with F
but for example the torture mission, if you start it with T, you talk about smoking, speed, sugar tits, and a super funny line about M's wife... but if you start with M, then T is taking a dump behind a garbage container...
there are a bunch of missions like this with unique scenes that you will likely miss your first playthrough since you can only linearly see 1
-most missions have 3-4 small dialogue... i think most of us probably realize it changes but maybe not much. for example, on Heist 3 when F is ramming the truck into the securicar, he can say I think 4 or 5 totally different things each time you replay it. almost every mission is like this. when T takes off his mask in the prologue, he may say nothing, may say so you want to see the angel of death, or he may say if i die i want to do it breathing easily or something... again if you replay every mission 5 times you'll likely here at least 3 sets and maybe more for almost all small dialogue
-F can phone Stretch after Long Stretch, Hood Safari, and Lamar Down and they will argue
-if you Hangout with Lamar after most of the F and L missions, they'll have unique dialouge... same goes for many other missions
-in Hood Safari, you can escape either by yourself or with L/T (talk about gang colours), L/F (talk about Grove Street), or F/T (talk about M probably the best and longest one for story)
-there's a lot of unique character transition scenes that only occur at the very beginning of the game, after T's family is gone, during exile, etc... and they're super good for building the characters. it's hard to sum it up but if you like GTA V, next time you play it, try to always free roam a bit after every mission and switch characters 6-9 times just to see all the cool little unique things they do and say. also as I said try hangout or phone people after every mission.
-after Bring Down The House... if you don't rush down the hill to M's house and just stop the truck... T and F will talk for like 5 minutes about old crime life.... it's such a huuuuuge and cool talk and easily missed. many missions can have story pieces missed if you rush them but that's one of the biggest and easiest and beast
that's it for now... i started a new game my 7th time two days ago and i'm playing non-stop and i just passed heist 2.... and i'm trying to play it the most 'weird' that i've ever played it, totally different haircuts (M looks surprisingly awesome with a shaved head, big huge beard, a white t-shirt, and blue work jeans, and tattoos lol and same for F with the big beard, shaved head, and sunglasses he looks exaaaaactly like Rick Ross lol try it the big Spartan beard, shaved head or a hat, big black gold sunglasses.... it's big boss Rick Ross lol)