The British writer Sam Hughes, known online as qntm, has this tip to share on his SCP Foundation page: Rings true to me.
I think he makes a good point, actually. He's not targeting authors who actually write epics. He's warning the ones who conceive epics but never get their epics actually written to maybe focus on a small part of their epic instead, and to write that individual story. It's easy to bite off more than you can chew. How many times have we seen this discussed on the forum here? Somebody who is 'planning' a huge epic, with 10 volumes, etc, but can't seem to get started because they don't know WHERE to start—and they ask the forum to 'help.' I reckon this is what Sam Hughes is addressing with this tip. He might be referring to 'chapter' as something like 'a chapter in your life' or something like that. At least that's how I interpreted it, although it's a bit weird to call it a 'chapter' in this context. But. Anyway.... I'd say take from this what you will.
Epics are always multiple story lines that meet in some way to move each other in some respect. Writing one story line at a time and then merging them seems the best way to approach such an undertaking. Not that I ever have or plan to. ETA: Cronin does that in the second of the Passage trilogy for example. The minor characters in the first book are fleshed out, meet in unexpected ways in the second and are used to move the plot in the third.
To a great extent, that was what I did with The Eagle and the Dragon, which at 550 pages might be considered an epic. But basically there was some major crisis that had to be overcome for every four or five chapters, each of which were substories. In a sense there were multiple "three-act plays" all building toward the central crisis, then multiple plays again toward the denouement. I never really concentrated on the overall story, just the current crisis for the chapters I was working on. And doing much the same on the sequel