Barring large scale war, how often did nobles fight one another?
I'm not talking war, necessarily, but did land change hands outside of warfare?
I'm curious what sort of stories to run dealing with combat in a saga set on the Dordogne, where one companion has recently become a Chevalier Banneret, and now has to deal with neighbors and vassals and so on.
Vrylakos
A lot! Raids and stuff. Free cities did that a lot too; right now (in 1220) there's constant bad blood between Venice, Pisa and Genoa.
Sure! Marriage and inheritance. That's how the Holy Roman Empire has just annexed Sicily, for example.
"Bella gerant alii, tu felix Austria nube!" ("Let others wage war, you, prosperous Austria, marry!") was the unofficial motto of the Habsburg dynasty two or three centuries later.
Also, at those times they did not have a computerized database of who owned what since when. There were tons of disputes about whether fief X should belong to noble Y who had inherited it from his father's father, or to noble Z who was the son of the daughter of the elder brother etc. etc. Basically, if you want a piece of land, and have enough political/military might to grab it, finding legal justification for why it should belong to you is really easy. Push come to shove, one can always resort to forgery as the Church did (though this may not true in Mythic Europe)! This is true at virtually any scale, from kingdom to hamlet.
Basically it can be as tough for the companion as you want. Theoretically, your liege should protect you, but if your liege is weak, or you are attacked under some wild pretext by another of his vassals whom he favours, he may well order you to ask the attacker's pardon and yield some of your lands as "reparation".
Where? It depends a lot on if the king ius powerful enough to subdue his barons, like say William I, or if he's too weak, like say, King Stephen.
An example of this was William Marshall's father. He started building his power block by taking manors and fortresses of others during king Stephen's reign. He did not even search for justification as far as I have read about him. He just got there and exercises violence to evict the current ruler under one weak justification or the other. This was really easy at the time: you just said you were a follower of one of the claimants (Maude or Stephen) and attacked your neighbor that was a nominal supporter of the other claimant. However, if you tried it under more powerful overlords that could mobilize powerful forces against you having a secure backfield, things could get MUCH worse for you. 
Insecure political arenas are always better for the small players to do as they please.
Cheers,
Xavi
The location we're playing in is a small castle on the Dordogne. The game has developed such that the King of France is unanointed, and the King of England is agitating to overthrow him. The players were caught right on the borders of this issue, but right now there is a lull in the conflict.
The companion is a vassal of the Viscompte of Turrene, in turn a vassal of the Count of Perigord, I think. Above them all is the Duke of Aquitaine, and theoretically the King of France, whom Henry III does not regard as a true king.
In Aquitaine, after Elanor dies, you can just go ape if you are fighitng non-Aquitainians. Indeed, doing all you can to make the French fight anyone else, and signing on with that anyone else, is basically your patriotic duty.
If you try a war -within- Aquitaine, then all of the other Aquitainians, who see unity as the only reason they aren't paetr of France, make an example of you.