This idea started with a ficlet included in my collection "All in One Sentence", and it's also the first piece of this series.
I apologise for any mistakes you might find, English is not my native language.
She let her hair grow longer, he had noticed one day. He told her it was very becoming and she just looked at him for a long moment as her eyes grew bigger, her lips parted and her mouth hesitant.
What he had not told her was how fascinated he was by that strand of hair that would fall down no matter what she did to keep it in place. Before he knew it, he had found himself engrossed in watching her tuck it behind her ear in irritation, only to see the unruly one break loose the moment she moved her head. Her long callous fingers moved almost unconsciously now as she mechanically trapped it time and time again, unaware of his gaze and the amused smile gradually settling on his lips, green eyes gleaming at the sight of the rebel lock slowly slipping away once more as a voice inside his head exclaimed "There it goes again!"
Author's notes: So basically this came out while I was writing a one-sentence fic; I had this image of Jaime looking at her in amusement but also with tenderness, being engrossed by her without even realising it. And then out of nowhere I thought about this passage from North & South by Elizabeth Gaskell, which is one of my favourite books and mini-series:
She had a bracelet on one taper arm, which would fall down over her round wrist. Mr. Thornton watched the replacing of this troublesome ornament with far more attention than he listened to her father. It seemed as if it fascinated him to see her push it up impatiently until it tightened her soft flesh; and then to mark the loosening — the fall. He could almost have exclaimed — "There it goes again!"
And I felt inspired to write something based on that - and I wanted to include John's almost "There it goes again" because I had this image of Jaime thinking the same as he looked at Brienne doing whatever I decided she would be doing. I was searching for something to replace the bracelet, and my mind thought about Brienne's hair, since lately I've been thinking I would like to see Brienne with Gwen's current hair length.