Just like people, all characters have (in theory) past memories and experiences, some of which impacts their present and future.
Writers are often advised to keep the story moving forward, to add just enough back story sprinkles to give the reader only the most immediate and necessary information.
But why is that? Don't "real" people reminisce? Can't characters do the same?
Well, yes, of course.
But think about it. When is the last time you recalled, in complete detail and word for word, entire long portions of your life?
Yet that's what writers sometimes do with back story. They dump large amounts of information at the reader in the guise of sharing, looking back, or monologing. Even Henry and Kellen, with their penchants for storytelling, only told one story at a time.
What if your character is sitting by the fire with a warm mug of tea and musing on the past?
Well, most of us recall precise details in snippets, some portions clear, some hazy. Furthermore there's probably enough action in the background to root the reader in the present: the crackling of the fire, the gong of the grandfather clock, the cat that jumps on the couch.
Back story, when skillfully woven together with current events "feels" natural and part of the world you're creating.
How often back story flashbacks ccur depends on the extent the past is affecting (needling, inspiring, etc.) your characters. Flashes of back thought can happen fairly often. They just don't, usually, happen in large exact thought. It's not realistic.
The same is true for two (or more) characters talking. The person providing the information may dominate the conversation, but not for whole pages. Again, it's not realistic.
Unless you have a reason inherent to the plot to do otherwise. In that case, disregard the above. :)