The target must be in line of sight. Select spell Obvious really. Neutral spells have no effect either way. The lighter the colour the name of the spell is, the higher the chance of it working. The overall state of the world is indicated by the number of symbols on the main option menu.
Real creatures have a chance of failing, but are unaffected by the disbelieve spell.
Other than that the two work the same. If you are player one then you get to cast your spells and move first. Some spells need to be cast at a target - attack spells need an enemy creature, create spells need a square adjacent to your wizard. Undead creatures can only be attacked by undead, magic weapons or attack spells. Computer places approx 8 magic trees near your wizard.