I wrote in the headline "What kind of art tool is needed" to make an illustration, well nothing special to be honest. Its your own choise. Its not in the tool itself to begin with. On the other hand its descided when you get an assignment. Your client mait have a preference regarding style, material and technique. If you have totally free hands to illustrate after your own personal choise of preference then just go create in whatever way you prefer and that you see suite the specific subject. Again when it comes to comissioned illustration you are very much in the hands of your client, meaning they useally like to have a say in the final result. Its in the nature of working together with other people.
That´s what differ from when you work freely as an artist doing your own personal work. But again... what now happens is that you as an artist is rarely that free that you are able to create with no hesitation. As an artist you have your own inner "ghost´s" and wall to conquer. As well as your willingness or un-willingness to listen to society´s moral aspects. You mait start to think: "what will mom say, what will others think, and is this realy art" and so on.
Even though you mait be totally free from any outside influence you have to consider the material and technique you choose to work with. And also the place of making the art comes in to play, as well as the future home of the artwork if it comissioned.
So the question on what tool one should use or if the art is made absolutly free is a question that could be asked more than one time. Especially when you get involved with another person.
The only time an artwork is made absolutly free is when the artist comission him/her self to do the piece.
Illustrator & artist