First, has anyone else tried playing a build who operates on an honor code?

I'm sorry I keep coming up with new ideas (curse you, Sheogorath for the creativity!), but I have pondered a Sniper build.

First, he's a male Bosmer, whose primary skills are Stealth, Archery, and Light Armor.  His secondary skills are Restoration, Blacksmithing (Steel, Elven, Advanced, Glass, Dragon, and Arcane) and Alchemy.

I figured out an honor code for this sniper:

One shot, one kill; avoid wasting shots

It is dishonor to murder a sleeping target (that one will make the Dark Brotherhood quests more interesting.)

Do not kill an animal unless you intend to eat it (that's a Klingon proverb, BTW)

Stealth is your native language.

BTW, I've beaten Alduin as a Wood Elf archer before, just never using a RP like this.

Notice his honor code says nothing about stealing or killing (except for murder of sleeping targets.)

Also, I plan to do this build, first on Apprentice, then Adept, then Expert, then Master (I've done several thief/assassin runs on Novice, and find them boring.)

What do you think of conjuration and illusion being role-played as an extension of Speech, like using a magic item to access Oblivion and persuading Atronachs, familiars, etc. to work for him?

All spells in Illusion but Muffle and Invisibility should fit, as you're 'persuading' people to chill or join you. Fear seems like a bit of a stretch, especially as your character doesn't sound physically intimidating, but it could work.

Conjuration is tricky, while summoning itself is hard to RP as an extension of speech (necromancy in particular), using the 'Command _' Spells are a perfect substitute.

What's a good motivation for a Dark Brotherhood assassin to delve into caves all the time?

Pursuing a leadthat his/her target was seem venturing into the cave?

Really great idea, but I'd like to have an excuse to scour every nook and cranny of Skyrim with a universal motive that isn't too specific.

Perhaps your character simply needs a reason to hone his/her skills?

Maybe, when you aren't actively working on a contract kill, you stay sharp by "exploring" caves. They give you a chance to practice sneaking past the inhabitants, "assassinating" others, and finding some loot while you're at it.

Thanks Castle!

Even assassins have a hobby. Do they use a crafting skill? Rare ingredients can be found in caves, along with ore and soul gems.