Baldur's Gate Expanded Edition also provides optional features of Multiclassing and Dual-Classing.
MULTICLASS MAGE/THIEF
A multiclass character can choose a combination of two classes that the game provides, such as mage/thief, fighter/mage, cleric/thief, etc., and then use the abilities of both classes at once.
This might sound like a good idea at first, and we started out experimenting with a multi-classed Mage/Thief, but in the end decided against continuing:
Xp in a pacifist run is limited to what can be gained by completing nonviolent quests (and a few very minor gains from things like memorizing spells and picking locks), making XP extremely scarce.
The problem is that using a multiclass character automatically splits the experience points gained equally between the two classes. This means it takes roughly twice the amount of experience to level up as a single class character, but without any of the many advantages of having two characters instead of one.
A Mage/Thief multiclass pacifist run is still very doable, but demands higher levels of xp whether you go solo, or join forces with other characters. (And if you decide to bring another team member into your band and intend to share xp with them, it means level-ups take even longer still, because xp won't just be spread two ways: Your share as a multiclass character will also split your own 50% share of the xp acquired between your own multi-class characters' two classes.)
NPC RECRUITS VS MULTICLASS
It's also worth noting that Baldur's Gate provides you with a lot of chances to recruit interesting NPC allies from various classes and alignments (who can also be customized, including changing their portraits).
This also diminishes the advantage of multiclassing and spreading one's xp out between two classes in a single character.
NPC allies who join the party can also be temporarily removed from the party at almost all times if you don't want to share XP from quest rewards with them, and then re-recruited right afterward in most cases, so that all desired XP goes directly to the player character (or 1-2 selected favorites, at the player's discretion).
For example, being a player who favors the enchantment and thieving skills:
Our Enchanter was joined near the very beginning by Imoen (a thief with 18 dexterity, the highest score a human can have when playing a thief), forming the ideal team of an Enchanter and Thief from near the start of the game. (If we'd multiclassed, we'd have had the spread of the same amount of xp split in the same way as with two characters, but only have been able to carry half as much treasure around, among other disadvantages.)
In our other run, our halfling Thief character originally started out on his own, but while exploring the Nashkel Mines, he ended up rescuing a saturnine enchanter named Xan from a cave, and relying on the latter's "Charm," "Color Spray," and "Invisibility" spells to more efficiently solve numerous key encounters. There were also several critical points where Xan was enlisted to use non-lethal wands to acquire items our Thief could never have acquired himself...
In both cases it ened up paying off to recruit the help of NPC allies, regardless of how you chose to distribute the XP by strategically adding and removing active party members at key points.
DUAL-CLASSING
Dual-classing is a feature only humans have, in which they can choose one class until they're leveled up to satisfaction, then change their class to a second one. They will never be able to advance further in the first class, but after reaching the same level in their second class will be able to use the abilities of both.
This is a viable strategy, though we ourselves decided against it for various reasons:
The biggest problem with it is that dual-classing causes players to lose the ability to use their first class's skills until they even things up in their second class (after which they continue to level up in the 2nd class, and regain the abilities from the first, even though the first class does not advance).
This could be tough for a pacifist player during their first run, since XP cannot be gained using violence, and an awkwardly dual-classed character runs the risk of becoming troublesomely handicapped in a pacifist run (since "grinding" by killing things to rack up XP for a level-up is never an option).
Prior to having already won a pacifist run or two through any specific game, it would be difficult to predict exactly which skills will be needed in which situations.
Even if you end up falling back on NPC allies for help (e.g. recruiting a thief or mage when you lose your own abilities before leveling up in your 2nd class), the dual-class character will also have had to "start" over after reaching level 2, and so be less effective than any character with a single class for a long time.
DUAL-CLASSED MAGE-THIEF
One viable build if you wanted to dual-class might be an Enchanter-Thief (or general Mage-Thief), starting as a magic-user long enough to cast the "Find Familiar" spell and then immediately dual-class to a thief who would later be able to cast charm spells, color spray, sleep, etc.
This is more or less viable, but since dual-class can't be used until level 2, we decided to not bother with it, since we were happier with our Elf Enchanter or Halfling Thief, who both got additional magic and stealth bonuses in their own classes.
If you had the patience to level-up in the Mage class long enough to get the invisibility spell, the build would be even more viable— but our take was that this would take too long build up to, since it takes a lot of XP to get that far, and your Thief (after switching classes) might end up being frustratingly underpowered unless you start maximizing their stealth skills from the beginning of the run.
