Well, it's definitely a tactics-intensive game if you want to play it that way. And in some boss battles you really do have to play it that way. You'll party-wipe if you just charge in with swords and spells blazing. How deep you get into the combat depends on how much you want to micro-manage using the pause key.
Ambushes of a sort are possible. You can have party members hold position on one side of a doorway, then send a controlled character through to lead enemies back into the room where you're party is waiting. Outdoors is a little trickier because you go into combat mode at fairly long range, although that helps archers and spellcasters. One nice thing, although it's a bit of a cheat for the player, is that you can spot and target most enemies for a first strike, before they'll notice you. So it's possible to scout out the battle before it starts, and look for the best tactical position.
Remember with this type of game you can very easily swap control between the different party members. I'm playing as a mage because I think it's ideal for a tactical overview and control of the battles, but I'll often switch to the Rogue in the party (when there is one), for initial scouting, or to make the first strike and get things rolling.
There are opportunities to use diplomacy, in the form of threats or persuasion. I haven't paid much attention to how this works in terms of your stats (dialogs are always based around your main character as the leader), and it probably also depends on where you stand with all the various conflicts between factions. Anyway, I've been able to use diplomacy in dialog choices to avoid a fight, or achieve a goal through persuasion that could only be done that way.
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