For Mack, it was easy to pinpoint just when he took an interest in Fitz. He'd never actually spoken to the guy but when Coulson spoke about him and what Shield stood for, he knew he wanted to help him. It is easy to fall for him after their first few interactions.
For Fitz, nothing was ever that clear anymore. Sometimes he thought it was gradual; like how Mack helped slowly tear down the walls that he and mental Jemma built. Other times it was hard and heavy and intense, like the time that Mack snapped at him like no had since before he woke up, yet defending him in the same breath.
When he thought the burn was was slow, he thought of how he appreciated each and every effort the man made to make him feel normal, though Mack insisted there was no effort at all. Still, he appreciated his new friendship with Lance all the same. Held on to the fact that even though his brain and his mouth no longer connected properly, Mack listened and heard. He thought of how he every time he smiled at him, it wasn't placating or pitying, it was playful and kind.
When he thought it was fast and hard, he thought of the first time Mack called him Turbo; how, if things weren't so dire, he would have stopped and, at the very least, acknowledged that his stomach flipped like the first time Simmons smiled at him.
Or maybe it was when he got a good look at those arms.
Whenever it was, he was in deep by the time, Jemma returned. He was as comfortable with Mack as he was with her. Lance had even taken to calling them FitzMack, though only to annoy them and never around someone who was here from before. Despite all the hurt she caused him, he still managed to feel bad. That's why he always jumped back awkwardly whenever she found them huddled, working on a project. It wasn't even intimate, but it had definitely been a FitzSimmons thing.
Leo was surprised when Mack came back from the garage, greasy and sweaty, saying that he just had a talk with Simmons and that he liked her. He had been less than sunny towards her since she arrived, though, he liked to think that only he received the sunniest side of the mechanic. Apparently, they came to an understanding about a lot of things; most of those things pertaining to him.
Leo pressed him for details but he was as firm as his stature. He only gave up that they understood each other, and that if he wanted to know more, Simmons was the one to ask.
He was extremely vocal, however, about how she insisted that he do whatever makes him happy with whomever made him so. Mack's imitation of Jemma was abysmal though, and Fitz made sure to tell him that.
Or he would have, if Mack hadn't kissed the sense out of him.