How did people from different classes interact with one another in medieval europe?

Medieval society was rigidly hierarchical, with everyone expected to know their place. Most people believed that the division between the classes had been ordained by God; in a deeply religious age this was widely accepted as the natural order of things. To a large extent, the class into which you were born was the class in which you died. In the days of feudalism, peasants were pretty much tied to the land. They lived and worked on estates belonging to the nobility, to whom they were duty bound to give service. If you were a peasant in those days you couldn't just up sticks and seek better opportunities elsewhere; you owed your subsistence, your whole way of life to the lord of the manor and so you wouldn't be allowed to live anywhere else.
Although relations between the classes tended to be quite deferential, social disorder was still quite common in Medieval Europe. Peasant revolts took place with frightening regularity, often in the wake of a failed harvest. Desperate peasants would take out their anger and frustration on their alleged social superiors and their property—looting, destroying, in some cases even killing. Order was often quickly restored, but not before huge damage had been done.
Feudalism was largely destroyed, not by peasant revolts, but by the Black Death. This deadly plague wiped out around 60% of the population of Europe. It was no respecter of rank or wealth and many nobles died during this unprecedented catastrophe. With the ranks of the nobility decimated by the plague, society changed dramatically. A shortage of peasant labor meant that workers on the land could command higher wages and break free from the control of their lords and masters.
Medieval society, though still hierarchical, was now much more fluid. A wealthy middle-class quickly emerged, taking advantage of the economic opportunities inadvertently provided by the massive reduction in population. Though the population was much smaller, demand for basic goods remained high, and the rising middle-classes such as merchants and traders made huge fortunes from supplying them.