For such large games, there are usuall several phases of development that can't be blended perfectly even under an agile framework. Theses are:
- Concept - Ideas are considered, prototyped, and thrown out quickly.
- Pre-production - Ideas, winnowed through the concept phase, are built upon and refined.
- Production - Multiple levels, characters and other assets are mass-produced based on what was discovered in pre-production.
So here is the graph I jotted down (I love Omnisketch on the iPad):
What this communicates is the flow of development (the green line) from concept, through pre-production into production. This flow can be for the entire game or a class of assets (such as levels). The goals for the team and stakeholders moves from "correctness" (making the right game or asset) to that of "efficiency" (making it as low cost as possible). Each goal guides how we do the work, either using spikes in concept development, Scrum in pre-production, or mixing in lean practices such as kanban in production.
The blue box in the lower left represents "total certainly" in our requirements and technology, which is fine if your mass-producing widgets, but is never a state we're in making games.
I find this useful in explaining that the work chooses the methods, not the other way around.