In single player, dispatching the human opponents (Tuetonic Knights and Society of Leopold) with Theft of Vitae results in zero experience points gain.
Sometimes transforming yourself into a wolf is the only way to effectively attack a warghoul or vozhd. When you are in wolf form, the enemy cannot lift you off the ground.
If you read the letter on Father Leopold's desk before turning over the blood vats, Christof will say an extra line of dialogue when the Advancement window appears. Use Cloak of Shadows to hide from Leo while you read the letter.
If a single knight or cop comes after you for something you've done, you can make him leave you alone if you bite him and then release him. Once he is bitten he runs off in fear. This doesn't work if he has buddies to back him up.
Often times the best way to beat a difficult opponent is to possess a character that the opponent is not focused on and let the AI do the work of healing your other characters while you beat at the enemy from behind.
In the Silver Mines, save all the holy water you find for Ahzra. Holy water doesn't work on ghoul creatures (like warghouls or szlachta).
At the end of the Silver Mines section, Ahzra is easily destroyed if you throw holy water on her first.
To hide yourself completely from others in a multiplayer game, locate the perfect corner of the map where you can get in really, really close to the wall, then use Earth Meld.
To prevent the dreaded "fumpfblumen" error (it happens in Multiplayer - your character gets corrupted and she/he, as a result is somehow renamed Fumpfblumen), go into your Crypt folder and set your characters to read-only (or back up the folder).
Don't go running into rooms full of enemies. Take one character in to draw the attention of one or two foes, who will then follow you back to where your coterie can close the trap on them.
Even if you're against cheating in any form, the console cheat is very useful for storytelling.