You need to assign points to "Move Silently" as well as "Hide in Shadows". It is quite possible that a B/S attempt will fail even if you have used a potion of invisibility because your intended target will "hear" the attack. Always attempt to be behind the target but assigning points to M/S will increase your success rate. Move Silently is often overlooked but it's important. The only skill that you can totally disregard for Thieves is "Detect Illusion", it's not worth assigning points to, I've never put a single point in that skill and of all the Thief skills that is the one that you will never/very rarely use.
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It's bleedin dark down here, slippery too.
Man, that's a pisser.
