I'm sure I've posted this earlier in the thread, but I might as well say it again. BG1/2 were based on 2e rules, which were NOT turn-based. They were simultaneous movement, with initiative-based combat/action resolution. Turn-based was introduced in a late-2e optional expansion, and then in 3e.
lol...no 2nd edition was very much "turn-based", it just used different syntax to describe the "rounds" of combat. Functionally, it was still turn-based combat.
If an encounter escalates into a combat situation, the time scale of the game automatically goes to rounds (also called melee rounds or combat rounds). Rounds are used to measure the actions of characters in combat (or other intensive actions in which time is important).
A round is approximately one minute long. Ten combat rounds equal a turn (or, put another way, a turn equals 10 minutes of game time). This is particularly important to remember for spells that last for turns, rather than rounds. PHB CH9 The Combat Round
In real life, combat is one of the closest things to pure anarchy. Each side is attempting to harm the other, essentially causing disorder and chaos. Thus, combats are filled with unknowns--unplanned events, failed attacks, lack of communication, and general confusion and uncertainty. However, to play a battle in the game, it is necessary to impose some order on the actions that occur. Within a combat round, there is a set series of steps that must be followed. These steps are:
1.The DM decides what actions the monsters or NPCs will take, including casting spells (if any). 2.The players indicate what their characters will do, including casting spells (if any). 3.Initiative is determined. 4.Attacks are made in order of initiative.
Furthermore, BG 1/2, both very much used turn-based mechanics behind the scenes to process attacking and spell-casting.
Combat is divided in rounds and turns. One round equals 6 real seconds, and one turn equals 10 rounds or 1 real minute.
1 round = 6 real seconds = 72 game seconds 1 turn = 1 real minute = 12 game minutes 1 game hour = 5 real minutes = 5 turns 8 game hours = 40 real minutes = 40 turns 1 game day = 2 real hours = 120 turns Baldur's Gate Time System
Combat is based on rounds. Rounds are periods that last for six seconds. The Speed Factor of a weapon determines when within those six seconds a creature wielding that weapon hits. The lower the number, the earlier the hit. A Speed Factor of 0 means that the creature hits at the start of the round. A Speed Factor of 10 means the creature hits at the end of the round.
Attack animations take 1 second to display. If a character attacks with a two handed sword (Speed Factor 10), the game starts the attack animation 5 seconds into the round so that it finishes (and the hit is delivered) just at the end of the round.
The game also rolls a random initiative factor each round so that even with a worse speed factor, attack can come first. Baldur's Gate Speed Factor
So interrupting your melee fighter or your spell caster, didn't allow them to immediately make a new action in that combat "round", they had to wait depending on the round initiative, and the casting or weapons speed before they could attack again since the combat was still being processed in terms of combat rounds.