Roahin: To have your Fighter dual class into a Thief you need a MINIMUM Strength score of 15 and a Dexterity score of 17. Also, you can't be Lawful Good aligned.

Note that with dual-classing, it's a one-way street. Once you start taking Thief levels, that's it. You can't go back to taking more Fighter levels. Make sure you have everything from Fighter you want before moving on.

Roahin: There are a few schools of thought on when the best time to dual-class away from Fighter is. Here are the things to keep in mind:

1. From levels 1-9 the Fighter is getting 1d10HP every time he levels. At 10 and after, classes no longer roll, so these HP gains are very important.

2. At levels 7 and 13 the Fighter gets an additional 1/2 an attack per round. There aren't many ways to increase your attacks per round (referred to as APR by most of us), although the time it takes to get to Fighter 13 is not insignificant.

3. You get Weapon Proficiency pips (points) at every third level of Fighter. 3, 6, 9, 12, etc. These can be quite valuable when you dual-class as the other classes acquire them far more slowly and the Fighter can go beyond just one or two pips in a weapon type, unlike every other class.

I find the most common level people dual-class at is 9. This gives them the extra half-attack, all good HP levels, and ends on a proficiency pip level. 7 is quite common as well, especially for people impatient to begin their journey on their new dual class. 13 is the rarest because it represents so much time invested, you're usually halfway through BG2's SoA campaign by the time you've hit Thief 14 and gained the benefits from dual-classing so late.

People don't usually dual class before 7 or after 13 though.