Really love the scene of when Riza stops Roy from killing Envy, because that's the one moment that clearly, blatantly shows just how dependent the two are on each other.
Riza had an outright suicide plan drafted if she did go forward with killing Mustang. Roy was ready to give up his life and die at the hands of his lieutenant if she deemed it right, but he only prevents it when he realizes she'll die if she does so. It's that moment that the viewers understand: their lives hinge on the other.
This isn't a romeo and juliet kind of situation. It goes far deeper than just "I love you too much to live without you" on either end. They both need each other. They are each other's vassals for a brighter future. They both consider themselves damned and unforgivable to the deepest core of hell, yet place their deepest trust and their hopes for the future in the other. Roy is the man that carries Riza's heaviest burden and her gentlest ideals for the world. Riza is the woman that is Roy's rock; the constant in his life, the one who shoulders his fragile ideals and anchors them when he strays. Their lives and burdens, goals, naive ideals and their dreams are so deeply and intrinsically tied with each other; they are the halves to the other and cannot be whole otherwise.
There's no heroism or vainglorious martyrism in either of their actions in this scene. It is the very intrinsic, deep human need to preserve the life of the one they deem the most important.
