Reposting a reply to someone who asked for tips and advice running a Cthulhu Dark Ages game. These were written with the idea of a medieval mystery game in mind, but I think they apply equally to pretty much anything trying to capture a medieval vibe.
1. All institutions are persons. The level of bureaucracy and institutional inertia we're used to in the modern era could only be *dreamed* of. The rules are *never* blindly enforced, and the systems in place to argue about them are incredibly complex compared to our expectations.
2. Labour is expensive, and preserving it is everyone's reflexive top priority. As much as the life of a peasant seems degraded to us, the entire system exists to keep people able to make food, and the margins for error for even a king are razor-thin compared to now.
3. Points 1 and 2 mean everyone is very aware of their debts and obligations in keeping everyone around them alive, and their reliance on the fulfillment of those debts and obligations by others. If one person in a village dies, it's everyone's business.
4. The world is alive. This might seem odd, since we're not used to the idea of medieval Christianity as "animistic," but the reality is that the medieval mind treated the entire cosmos as hylozoic — they describe physics, geology, weather, everything in terms we would reserve for plants and animals.
5. Life is mysterious, and mysterious forces surround everything. A tree grows from an acorn; a child from its mother; a corpse births the worms and flies of corruption. It is not weird to assume that a child born under a particular star-sign will have some traits, for the fixed and wandering stars scribe the cycle of the seasons themselves upon the heavens.