[Darksea]

At level 10 Arcane casters and Bloodmages gain the ability to Counterspell. At level 10 I believe all casters counter in the same manner which is to dissipate the spell. At higher levels this ability gets upgraded with different classes affecting the spells differently. As I recall Druids can turn them into heals, Psions into energy and Sorcerers reflecting the damage.

At level 10 when my casters first get the ability I find some low level casters, engage them and then let them cast on me, countering the spells they cast to raise my skill.

When the NPC begins to cast a spell you will see a timer that displays how long before the spell goes off. Generally speaking the longer the cast time the more you want to counter it. In some cases you may elect to neglect fast casting spells in anticipation of a worse when or the NPC casting a heal.

As I recall ( have been playing melee recently ) when you are successful the ability will be immediately activateable ( is that a word?). If not then there is a refresh time of 20 to 30 seconds. To be successful you need to be able to identify the spell. You will know you have successfully identified the spell by the counterspell icon lighting up. I believe there is a small chance of being successful even if not identified