If you're playing a Thief/Mage, or the like, and cast Time Stop, does that mean you can just go behind a enemy and automatically backstab him? Does that work?

yes. but all damage is dealt after timestop ends. and btw : with your thief HLA you can lay a timetrap, so no need for the spell if you just intend to backstab.

But even though the opponent can't do anything about it, I think you still have to successfully hide in shadows before backstabbing. The time trap does work, but I believe it is shorter duration than time stop. You can get three spells off in Time Stop (so I guess than means it lasts about 3 rounds) but I think Time Trap only lasts one round. Still, it's probably better to use the trap than use up one of your valuable level 9 spell slots on a Time Stop where all you'll be doing is backstabbing.

If using a mage/thief. Cast mislead before setting the TS trap or casting the spell this will allow you to backstab over and over without needing to hide.

If you want the best spell-buff combination before a Time Stop to maximize your backstab damaging potential...

Cast Improved Haste, Mislead, Tenser's Transformation in a Spell Trigger.

Cast Time Stop, Cast the Spell Trigger, backstab away.

Alternatively, you could skip TT, but it gives a HUGE boost to your THAC0, making sure that your six backstab hits connect.

If using a fighter mage thief - Stick in a critical strike after one or two backstabs and your laughing the rest of the way.