"This is a six part documentary. six parts. That's a lot," she said.
"It explores the clandestine days of Harry and Meghan's early courtship and the challenges that led to them feeling forced to step back from work in the royal family.
"It shows such intimate pictures in there. They really are shots that we've never seen before. Some of them are selfies, as you can see. Some of them are from that holiday I think that Meghan and Harry had early on in Botswana when they first fell in love, and some of them are clearly aimed at showing how they have suffered in that media spotlight.
"We thought we weren't going to hear anything more from Harry and Meghan that we hadn't heard before. But I think this shows us that we are going to hear something that we haven't heard before."
WATCH: Prince Harry and Meghan Markle's tribute to Elton John. Article continues after video
"It's going to be very intimate," she concluded.
Rieden also suggested that Harry and Meghan have "been planning this for a while" by the looks of the professional shots taken inside Nottingham Cottage.
It was reported in 2020 that the contentious couple had signed a $200 million multi-year deal with the streaming giant.
The contract was said to involve documentaries, documentary series', feature films, scripted shows and children's programming.
In a statement at the time, the Sussexes said they wish to create content that "informs but also gives hope" and to "share impactful content that unlocks action".
So far, Harry and Meghan have released Harry's Rising Phoenix - a documentary about the Paralympic Games - while Meghan's animated series named *Pearl* was dropped during development due to cost cuts.