This happens after the end of the manga/brotherhood. I don't have any one particular head cannon, so my stories won't necessarily line up with each other.
It was a one-sided love affair, if she had anything to do about it.
Riza Hawkeye knew she was precariously close to making it a mutual relationship at any moment, but, just as she was constantly on guard in her physical world, so she was in her emotional world as well.
In her physical world, there were many dangers. Many places to watch, many voices to listen to.
In her emotional world, there was but one threat: General Roy Mustang.
The man she has known, followed, respected, admired, pushed, guarded, shared her deepest secrets with, these past twenty some years.
He had obviously fallen for her years ago, though this belief did not arise from any particular conceit on her part. Merely, she knew him, and knew that he loved her.
He was to be the next person in charge of their country, be it fuerher, president or prime minister, as soon as her grandfather retired from the position. There was no fraternization among the military, and she would do nothing to jeopardize his future.
Therefore, she would keep their love affair one-sided, and always protect herself from falling in love with him. Even though, in every other aspect, she loved him above all.
It was a one-sided love affair, at least if he had anything to do about it.
He would not fall in love with Riza Hawkeye. She, being his adjutant, guide, conscience, commander of his soul, would not allow him the luxury of falling for her.
He wanted her, that much rang true. Every day. When she picked him up in the morning with drowsiness still clinging to her eyes and the sun glinting off her hair. When she brought coffee to him with her particular way of cupping her hands around the mug and inhaling right before giving it to him. Her empty threats to get his paperwork done (they both knew he had spent a lifetime of bluffing) inciting a fire in her amber eyes that made him acquiesce, just to make her turn her temptation elsewhere.
He wanted her differently when they were younger. She was a wisp when he first saw her and he gave her little mind. As she grew, so did their friendship and his affections.
Then, somewhere along the way, be it in front of her father's grave, or behind a sniper rifle pointed at his head, she fell in love with him.
Roy could admit, easily, that nearly any woman he met would fall for him. He was dashing, mannered, rakish, and good looking. He used it to his advantage, whether as a distraction, or for information, or even vain attempts at something resembling a relationship.
But, when Hawkeye fell for him, he didn't know what to do with himself. All attempts at other relationships ended for him. With the force of all the time they had been together behind him, he had been ready to collapse into her for eternity, read to accept her love.
She stopped him with just one look. One flash of those eyes and he knew, "No sir, I will never forgive us if this ruined it all."
Every day he was close to tipping. Tipping toward her, but never away.
They touched even less than before, ever since she had almost died.
When his sight returned, he feasted upon her visage, teetering toward her, just a little more.
But, he couldn't touch her. It would be the end of his resolve, and he had silently promised her he wouldn't fall in love with her.