One of the things I have observed with the Remake trilogy is the use of visual storytelling in depicting character relationships.
Regarding ZA in particular, there is a lot of visual storytelling, especially in Remake and Rebirth. The storytelling tool is effective for their story (tragic love story) because although they are not physically in the same space as one another, we (the player) are given visual imagery to show how they are still connected to each other. Things like parallel dialogue, symbolism and shot choices are used to depict this.
Here are some examples how dialogue is used:
REMAKE
When Aerith first meets Cloud after the Sephiroth illusion, she ask: JP "Are you okay? (paraphrase)
The camera shot, her head angle and the way she looks at Cloud is very similar (shot for shot) of Zack looking at Cloud and asking if he's ok in the AC flashback of him.
When Aerith gives the flower to Cloud, she says: JP "Its a memento of our meeting" (paraphrase)
In CC, Zack says this to Aerith when buys the bow for her.
When Aerith loses her balance on the Sector 5 roofs and Cloud catches her, she says: JP "How reliable" (paraphrase)
In CC, when Zack fights off the monsters, Aerith says the same thing to Zack.
When Aerith sneaks out to spend more time with Cloud, she says: JP "I want to spend more time with you" (rough translation)
This is one of the 23 wishes that Aerith tells Zack in CC.
When Aerith looks at Cloud's eyes and leans in closer, its a very similar shot to when Aerith leaned into Zack to look at his eyes in CC.
Also, the line Cloud says about the mako eyes is the same line Zack says to Aerith in CC (if not, its very similar - I need to double check this)
REBIRTH
At Aerith's bedside, Zack says: JP "Moshi Moshi"
Its the same line Aerith did to Zack in CC. Aerith does it to Cloud as well, however, I think there is more significance to Zack because ZA have a history of repeating each other's lines.
When Aerith says something about the sunscreen in costa del sol, she says : JP "Hyu Hyu da ne" - That's cute (rough translation)
When Marlene affirms the Aerith's feelings for Cloud, Zack says: JP "Hyu Hyu da ne" - That's cute (rough translation)
In the dream date, when Aerith takes the flower pin, she says: JP "I will always treasure this" (rough translation)
In CC, when Zack buys the bow for Aerith, she says the same line to Zack.
In the opening of rebirth, Zack tells Aerith as he holds her body: JP "I have returned back to you" (rough translation)
In the end of CC, the last thing Aerith tells Zack (as well as the last Aerith dialogue that appears in his DMW) is: JP "I'll be waiting"
In the LOVELESS play, the hero tells Rosa: JP "I will return for I know someone will be waiting" (rough translation)
All in all, its a type of storytelling that is easily missable because it relies on SHOW rather than TELL. If you pay attention, there are plenty of little and details and nuances that serve this narrative. Also, the dialogue parallels are missable if you don't play in JP because the translations are slightly different.