1) +5% if the hero and neutral creatures have the same alignment (good/evil – as in the calculation of Battle Spirit)
2) +5% if the hero and neutral creatures belong to the same castle
3) +5% if the hero's army contains the exact same creatures as the neutral army (regular and upgraded creatures are considered different)
4) +5% if the neutral creatures are regular creatures, and the hero's army contains their upgraded version (e.g., neutral creatures are peasants, and the hero's army contains militia)
5) +10% if the hero has the "Diplomacy" skill.
6) +20% if the hero has a specialization in creatures from the neutral army
And one more question (I hope I'm not being annoying): where can I edit the starting armies of heroes? (Not on a specific map, but generally in the game.)