Using a children's book project as an example, if I do that type of work for a publisher, usually they send me a complete book layout with the spots for the pics in place, and then I just create each one of those.
The creation process sometimes starts with me having 100% freedom to paint what I want based on the texts in the books, others the art director either sends me some examples of stuff they would like to see, or we talk for a while about some specific details needed for each painting; the mood of the pic, the time of day, what it needs to represent, the best colors, what we need to depict based on the story, how that image will affect the next one and what we need to do to create a balance in the narrative visually, etc.
As you can see all the input is mostly about technical details. Sometimes is very specific but totally unrestrictive when it comes to the creativity side.
There is never that thing, demanding that I need to paint that character in a totally specific manner, using only some very specific techniques or anything similar.
All the notes are based on what the story needs best to have it represented and then I just start to paint.

As I mentioned in another part of this site, I´m not one of those artists that work a lot with sketches to present to the client and so I just go through most of the pics on my list and paint them with enough loose details and enough editable layers to later do some revisions.

Within that process, I either send them to the art director or not. Most of the people I work for they know they get better results from me when they "take a chance" of letting me loose out there and so I don´t tend to have many back and forth moments while illustrating stuff. Other illustrators work a lot with pre-planning sketches and get to be more in contact with "the base" but I do work better when I´m left on my corner to just paint.
If I have a doubt about some objective in the pic or about some detail or some color, of course, I send my current thing to the client and then we talk about it and after that, I go back to my lonely painting process until I complete each pic.
In the end, when I have all the pics done, I go through each of them with the client and the art director usually requests some changes here and there, mostly remove or add stuff from scenery, change the light somewhere, or replace a detail.
It´s never about, an art director coming to me demanding me to revise a complete character for example having to redraw it from scratch to meet a specific type of line art or color style for example because we already talked about at the beginning regarding specific details.

That company hired me based on what I can do and all I can do is displayed on my portfolio, so what´s expected of me is replicating exactly what I do best over there (not focus on a private "vision" purely based on personal taste); and this means they all know that my style is that one seen on my website, so there´s never that thing that totally happens with self publish TYPE 2 clients where some people demand the illustrator to scrap most of his work on a page spread and replace it with stuff based specifically on details seen on the writer´s amateur sketches; for example because what he did deviated from "the author's vision". The one who pays the money...

The difference between a professional-client and a TYPE 2 self publish one is that the professional company aims for "their vision" by making sure they specifically hire an illustrator that already replicates in his portfolio all the type of details that coincide with the vision the publisher has for that project.
A publishing company does not just hire an illustrator because they´ve seen some pretty pics on the web and then demand that the artist only draws accordingly to the self-publishing author´s "vision" in the middle of the creative process without having a chance of putting all his best creative features to serve the project.

In fact, usually, the
TYPE 2 client does everything it can to prevent the artist from even using his own techniques, personal brush stroke styles, or anything that define his art because the client feels it does not fit what´s inside - the vision. A big no, no, and something that never happens when an illustrator works with a professional company.

That is what I mean when I try to describe the difference between TYPE 1 and TYPE 2 clients out there.
TYPE 2 only exists in the Self Publishing world and most of them feel so sure of themselves also because of "the love" they get online from other wanna-be authors out there which are exactly the same; ( all communicating through LIKEs on social media as if they are under a strange self-help pat-in-the-back auto-hypnosis thing ). The same, who then have no clue about how the professional creative world works but then at the same time are looking for a professional illustrator to illustrate their book.

To illustrate their book as long as that same professional illustrator agrees to start creating the book as if he was an amateur that is just starting and does not know a thing about what makes illustrated books work. In fact, the illustrator does not need to know because it´s clear the TYPE 2 client´s - vision - has all figured it out. What the artist needs to do is just to stop trying to control the author´s creative process, shut up, and just draw because it´s for that a TYPE 2 client clearly hires an illustrator. That is why many out there don´t even care if they get a professional or an amateur because in the end they just want their vision to appear magically at the end. And those are the books that never go anywhere.

This creates a really dangerous loop of people who believe honestly that remotely controlling the person they hired to draw their books is the only way (or the professional way) of maintaining their original writer's vision pure and will never understand that it´s not about how the images are painted but how they are thought about.
If they don´t allow the illustrator´s input to actually change what is wrong with their book then there is not much to say.
And many, many times, when it comes to TYPE 2 clients, the worst thing their book project can have is their "vision" for it because they will never let go of that thinking they are absolutely right and everyone else who disagrees is wrong. And those types are the ones which don´t want to listen to professional illustrators' opinions at all as they have all figured it out and if the illustrator does not agree it´s because he´s not professional enough to recognize they are right; and besides, they are paying and their mums and online friends say they are really talented too.