This week I'm going into some depth about those magic-users who get their powers through some form of client-patron relationship with a supernatural entity.
Some magic-users gain their powers through a direct connection to a supernatural source. This source is often a distinct entity, such as a spirit or a god, but this paradigm that also includes a source that is a collective entity such as the elemental powers that Warlocks derive their power from. These magic-users do not comprehend what magic truly is, or how it works; they are not akin to scientists or engineers. They are, at best, akin to technicians in that they know what their powers do and how they can best apply them, but they have no comprehension (outside of what their patron allows them to know) so you can expect them to master the practical employment of their powers, but be unable to innovate such that they can create new ones. They are users, not programmers.
The Witch and the Priest are the two most common examples of this paradigm in practice. In both respects, the magic-user enters into a contract with a supernatural entity. The magic-user agrees to act as the entity's agent in the user's environment in return for the entity's investment of power into the user, making the user's continued employment of his powers conditional upon fulfilling his patron's agenda (which means following orders). This is not a negotiation that goes entirely as the user intends, as the entity enjoys a massive power differential over the user, but the patron is rarely so aware or intelligent that it cannot be outwitted by an exceptionally cunning user. Whether this is because the entity is unaware, uncaring, or merely entertained by the user's attempt to get around the patron's constrictions upon the user's end of the pact is not that important; what matters is that he is not the source of his powers, and that he has some constraints on their use for reasons that are not emergent properties of those powers.
The Shaman and the Warlock also fall into this category, as are some Druid variants, but they are looser examples. The Shaman is chosen to represent his patron to his people, and the Warlock has a tie to the elemental planes that is neither his choice nor inherent to his being. Their powers are as ill-comprehended by them as the Witch and Priest comprehend theirs, but the constrictions upon those powers are so much looser; this is due to the nature of the relationship between the supernatural entities imbuing them with power, compared to the two Occupations above. The Shaman is not a subordinate of his totem spirit; this is not a true patron-client relationship, but rather principle-agent (and in both directions; he represents the totem to his people and his people to the totem). The Warlock's relationship is more like that of an adoptee or a master-student pattern; the elements decide that the user is a good choice for their association, and the imbuing of power into it (and training in the use thereof) is their way of forging that bond. Unlike the others, it is unheard of for a Warlock to lose his ties to the elements; this is (rightly) seen as being reflective of their overall long-term thinking and general apathy vis-a-vis mortal perceptions of time and the importance of mortals' lives.
Another variant is the Tattooed Man and its variants. These magic-users are actually men-at-arms. Their magic powers are bestowed upon them by their patron, in return for servitude (in the case of slaves or indentured servants), yet they have no comprehension whatsoever as to how these magic tattoos operate. All that they know is how to activate them, use the effects generated, and manage the power that fuels them; they might as well be Vagabonds in all other respects. While their powers can't (usually) be stripped, as such, the powers that they possess are nothing like what proper spell-casters and the ritual-workers that typify Warlocks, Shaman, Witches, Druids, etc. are; their lesser power (by comparison) makes up for their lack of vulnerability to losing their powers. This category also covers super-powered individuals such as D.C.'s Shazam (i.e. Captain Marvel), and those like him (e.g. Buffy, some Magical Girls); they receive their powers, in the form of human physical augmentation instead of spell-casting, but are otherwise the same.
The Shifter is not, in the baseline sense, in this category. He slides into it when he makes use of his Occupation's ability to contact supernatural entities and strike pacts with them, at which point he's in the same boat (with regard to those gained powers) that a Witch is- but he has the notable advantage of knowing how magic works in a very academic and scientific manner, which allows him to turn the powers he gains from these pacts into the sorts of rituals and spells that he ordinarily uses. This makes the Shifter, if he is both sufficiently resilient as well as ruthless, a dangerous magic-user; he can make pacts, extract new powers wholly under his command from them, and then dissolve those pacts before restarting the process anew.
By now, you can figure out for yourself what other Occupations fit into this paradigm. Now that we've got this established, let's get into playing them.
Your man is a user. This means that the sort of deep and thorough examination and study of how his magic works--the sort of thing that a Ley-Line Walker, Shifter, or a Techno-Wizard possesses--is not present. He is more like the Mystic in that respect, and often has his powers manifest in a similar manner (in that they appear suddenly; the user intuitively knows what to do and how to do it); otherwise, his patron instructs the user after receiving the investment of power. He views his powers in the way that a Coalition soldier views his tools, and like that soldier he can (and often does) refine his technique such that he can innovate new applications for those powers. Yet, when faced with a situation that demands a knowledge of the workings of how supernatural power works he is as useless as a Vagabond. (Don't ask a Warlock about how to best exploit a Nexus Point; he does not possess the knowledge required to do so.)
Your man does not think of himself as a magician. He sees himself as a priest, shaman, etc. if his powers originate from a clerical Occupation (which includes the Witch; he's a cult leader), or otherwise as a soldier/warrior/artisan whose powers are but tools of his trade. He views magicians as the sort that Shifters, Ley-Line Walkers, Stone Masters, etc. are, and that is often due to his ignorance regarding the nature of his powers. He sees his powers as gifts, grants, blessings, etc. and not spells and rituals and his language usage will reflect this perception. He views the fact that those same Stalkers that go after "magicians" also view him as prey to be due to the supernatural power imbued into them by their source; it is, at best, part of the price for their powers and not proof that they too are magicians. (Stalkers, for their part, are just as likely as not to buy this line of argument; they aren't experts on what magic is or how it works either.) That his powers interact with the spells and rituals of magicians, by and large, doesn't bother your man either; power is power.
Your man's relationship with the source of his powers is the primary relationship in his life and it comes before all else. Maintaining this relationship is always top priority, and as your man is typically the client or agent of that source that always means being prepared to fulfill the obligations put upon him by that patron/principle. While, in play, this should not (typically) be the all-consuming thing that it could be it should never be out of your mind- because it is NEVER out of your man's mind. The price your man pays for power is servitude, if not outright slavery; you're playing the sort of man who is always on call, and so must be prepared to drop everything when duty calls- OR ELSE!
Running campaigns for these characters is actually really easy, in that it provides ready direction and content for your players to go with and work through. However, it works far better if you sit down while away from the table working out the nature of those characters' patrons/principles/etc. before you bring them front-and-center like that. I advise you to do just that; it's preparation time well-spent, and brings great returns on that time investment when you make it recur time and again.
On a similar note, these characters are great NPCs- be they as allies or enemies. They come ready-made with motivation and means, making them great choices for external actors driving events in your campaign environment. Some are good even as minions, but most are better used sparingly as NPCs; keeping them rare usually works better for both verisimilitude and player suspension of disbelief. They're the leaders of movements, institutions, nations, and kingdoms; if you need a faction, employing one of these sorts of magic-users is a good choice. Enjoy.