How improvisational is magic?
Most spells and rituals have specific effects, and while you can roleplay the casting however you like (as long as you meet the casting requirement in terms of time and mana) the effect will always be the same. The Heal spell takes 30 seconds to cast and will heal one target to full health. What you do in those 30 seconds is up to you, but the effect will not change.
There’s some interpretation allowed in how the effect occurs. You might feel that your shatter spell makes an item crumble, or it might just give you one burst of strength enough to shatter an opponent’s shield. The end result is still the same though, your enemy loses the item you just hit.
The exception is that it is possible to make arcane projections (one shot ritual magic that does whatever you want) If you can get backing from the imperial conclave then it’s possible to get a new ritual put into imperial lore so that everyone can learn it and it will be added to the wiki as a standard ritual.
What different ways can magic and role playing effects be put on a person?
Rituals, Religious skills and the roleplaying effects of certain potions are the only ones that come to mind.
Priests can Consecrate (RP effect on everyone in the area) Hallow (RP effect on item and bearer) and Anoint (RP effect on target person)
Mages can curse people, or give them enchantments that often have RP side effects (both done via rituals). Potions often have similar RP side effects to the enchantments. E.g you might be extra tough for the next battle, but you’ll also feel overconfident for the same period of time, or you’re more resistant against poison but you’re also extremely hungry.
If there are specific spells how do I learn those rituals and how many spells are there?
Anything on the wiki is assumed to be so well known and researched that you could just find the instructions to teach yourself if you wanted. Magic is a skill like any other, some are self taught some learn from others but the end effects are the same. I think Graham has linked to all the important pages in his answer above.
What are the main differences from how the abilities of Religion and Magic?
A very interesting question. It is the current view of the empire that Religious effects are not magic. Not everyone in the game world agrees with this, but if you tried to say publically that priest skills were magic the Synod would be having a quiet word with you about heresy.
Mechanically, magic requires mana, Religious skills require Liao, a religious drug that is consumed during the ceremony. Bigger magic = more mana, bigger religious ceremony = more Liao.
In terms of game mechanics, religious skills tend to produce RP effects and occasionally protect against other RP effects. Magic has more effects that alter game mechanics, such as healing, paralysing, boosting farm production etc.