Because having the right position in combat is so important, you'll constantly be moving your characters slightly so that they are flanked as little as possible and flank as many opponents as possible. That said, there are many situations in the game where you'll be outnumbered two or three times over.
This is where crowd control comes into play. There are several different kinds of crowd control methods available in the game. Stuns are the most effective, as they shut an enemy down completely and they don't require as much movement on your part. But knock-downs also serve to take an opponent temporarily out of combat.
The most powerful forms of crowd control are those which handle more than one target, such as Morrigan's Mind Blast. Although the stun lasts only a few seconds, it is enough to do some damage. This is particularly true when facing numerous weaker opponents, as you'll be able to kill one or two enemies before the group comes around. Be sure to learn the range of any mass stuns in your arsenal, because failing to stun half a group because some opponents were out of range can lose you the battle.
Single target-stuns can be used at any time to take pressure off a specific character. They can also be used in chains to disable a powerful enemy for a long period of time. For example, if you have two shield warriors you can easily have access to one stun and one knock-down on each character. By chaining these together, using one ability just after the effect of the previous one wears out, it is possible to shut down an opponent for most of the battle.