1) Neither would ever admit it, but there were nights that Roy woke up feeling sick from a dream of sending Ed to the mission that would finally kill him; and Ed woke up thinking that somehow, someone had finally managed to defeat the undefeatable Colonel.
2) When Ed joined the military, he had shouted, insisted, sworn that he was doing this strictly, one-hundred-percent for Al, and he would never ever ever think of his commanding officer as anything more than a means to an end. Half a year later, he was blushing angrily and asking for an allowance.
3) Roy was never particularly keen on the vivid red coat that Ed seemed so attached to. It was much too bright, too distinctive and trademark Fullmetal Alchemist for Roy's liking. It occurred to him that if anybody wanted to track Ed down, it'd be laughably easy: With that completely indiscreet, neon sign of a coat, he was a walking target. But even Roy had to admit that it came in useful sometimes—especially being able to drag Ed back by the hood whenever he tried to stomp away.
4) It was more than just the Colonel's teasing that set Ed off so much; it was the look he gave him, the gaze that cut straight through all his defenses and forced him to spit out all of his secrets. It was the distant but disturbingly certain knowledge that Roy recognized the darker part of him, the side that Ed never dared to show even his little brother.
5) Edward Elric was not good at keeping secrets. If it involved his brother or anything else that he deemed serious to enough to guard with his life, he would do so. But any secret that weighed less than unquestionably essential was quickly disregarded, and any idiot could get it out of him, no problem. Unfortunately for him, Roy Mustang's ambition to use any measures to get Fuhrer was estimated to be Only Slightly Important. There were many times that the Colonel only just stopped Ed in time before he blurted it all to the wrong person and ruined everything. He just hoped that nobody thought it was odd that sometimes in the middle of Edward's sentences, the Colonel would casually slap his hand over the blonde's mouth and drag him away for an impromptu yelling-at.