Since all the guys have hair log enough to cover the backs of their necks… what would their reaction be to their s/o gently pushig their hair away to kiss the back of their neck as a little surprise? (… Gladio and Ignis would probably have to be sitting down for this to work with an average s/o pfft)

They’d definitely be sitting for these. 

Noctis:

This poor boy would jump a mile in the air first off. He’s a lot jumpier than one would expect. His face would turn red and he’d probably try stuttering out a question. Which would never make it out of his mouth, because his S/O would just laugh and walk off, confusing him. (He’d really like neck kisses though.)

Prompto:

Prompto would shiver at the feeling of cool lips pressed against his neck. He’s definitely not used to anything touching his neck, so he’s going to flush a very dark red, but smile in appreciation all the same and return their affection with a kiss to their cheek.

Gladiolus:

He’d probably hum in acknowledgement, not outwardly showing any surprise, and before they could step away pull them into his lap and kiss all over their face. They’d both laugh, and press their foreheads together. (The other make fun of them once they’re done for being so sweet together.)

Ignis:

Pressing a kiss to Ignis’ neck as a surprise, and then expecting a reaction, is NOT a good idea. (Or maybe it is depending…) He’d be the one to turn it into a game. They’d press a kiss to his neck, so he’d, later on, press a kiss to that small area beneath their earlobe and whisper sweet nothings to them. If they manage to pretend to be unaffected, they’d try and come up with a way to draw a surprised reaction out of him. (While he’d do the same.) The others can see whats up, but don’t mention it. It’s slightly entertaining watching them. Like watching a spy vs spy cartoon.