You can cast ANY spell with ANY character (you need a spell scroll and a potion for this).
1. Go to your inventory and right click on a potion.
2. Select "drink potion."
3. When you hear the sound effect (*gulp*) do not wait for the number below the potion to decrease, pick up the potion immediately.
4. Now while the potion is stuck to your pointer, click on an empty inventory slot and put the potion there.
5. Select a spell scroll and place it in the slot that was occupied by the potion recently.
6. Return to the game.
Now your character will cast the spell instead of drinking the potion.
NOTE: Any spell scroll will do. This was tested with healing potions only. Tested on version 2.5.26498. Might also work with other versions including Shadow of Amn but this is not tested.