Gas Mask War has a long and intense story, full of characters with their own stories. But I want an action game, fast and funny. I like the games (like Far Cry) that let the players the freedom to choose if they want to live the story or not. Long cinematics breaks that rhythm. So the flashbacks are a bridge between enjoy the story (without long cinematics) and enjoy the gameplay. (I based in Far Cry 4, Shangri-La Missions).
The players will unlock the Flashback Missions discovering Dove’s Diaries and War Diaries. These have the story of someone and will allow the players to travel into the time to live the story. Instead, to put a long dialog, cinematic or a short story that the players would have to spend the time to read it, I prefer that the players enjoy the story with an intense mission. The players will play the Flashback Missions whenever he wants and how many times he wants.
These missions will not appear on the map. The players will need to find the Diary first and then this will be unlocked one of the characters in the game menu (his Diary). Something like this:
(I based this menu design in the character selection menus of the fighting games like Mortal Kombat)