You start the game with two avatars in your party. Let's call them Roderick and Scarlett for now. Obviously when starting the game you can name and customize them. And yes, you can also play this game with two men or two women, it doesn't always have to be man/woman.
Roderick and Scarlett are the true heroes of this tale, and they'll always be in your party, but should you feel the need for reinforcements, you can recruit two extra party members.
Except in a few special circumstances, your party doesn't need to stay together. Sometimes the game will ask you to gather your party first, usually only when going from zone to zone, but our zones are very big (think Divine Divinity big).
But in one zone, including its houses, cellars, and dungeons, it means that you can control Scarlett to explore an underground cave while Roderick buys some fresh fish on the market. (Each hero's henchman will follow his master.)
Because Divinity: Original Sin features a turn-based combat system, this can lead to pretty interesting situations.
For instance: you switch to Scarlett, and in the cave, she encounters a friendly community of unfortunate ghosts, and has lengthy conversations with them, trying to understand why their souls aren't floating towards the Hall Of Echoes.
You switch to Roderick and, shopping around, realize he doesn't have enough gold to pay for his fish. So you try your hand at stealing. Lacking the necessary skills and talents for conducting successful theft, another shopper catches Roderick in the act and calls for the guards. Roderick is a bit short of temper and he's also on a mission to save Rivellon: there's no way he's going to put up with such foolery as going to jail, so he resists arrest and draws his sword.
Turn-based combat starts on the market, and several guards, as well as some market-goers that feel up to it, join the fight. Roderick has a high initiative, and in your first turn, you manage to conjure an earth elemental to even the odds a bit before any of the guards can even make their move. But there are a lot of guards…