This is an excellent post that I hope lots of people read and take to heart! If I may add a couple of extra notes;
1) If you have an idea for a character, but you don't have any sort of reference sheet, I implore you to commission a clear, standing reference FIRST before jumping into a big dynamic one-off piece. It may seem more exciting, but it's extremely hard on the artist to draw a dynamic pose and style for a character that hasn't been designed at all - AND not to mention designing a character is a whole task and set of skills on its own!
If I'm approached with a job like this, I charge extra for the work I will have to put into designing the character that is being put in the drawing, on top of the drawing itself. Please do yourself and your artist (and future artists!) a favour and get the reference sheet first. Not only will the design itself likely turn out better, but drawings with a proper reference will also turn out better!
2) If you approach an artist with a commission, please be upfront about all the necessary information. It can be somewhat frustrating when someone messages saying "I want a commission" with nothing else, no reference or description or anything. Explain briefly what you're looking for, and include any important references (or tell me upfront you don't have a reference) - that way I know immediately how big the job is and also how much to charge, otherwise I'll just have to ask you for that information anyways. Try to make the process smoother for us artists <3
3) I also really want to reiterate that commissioners learn what is useful and not useful information in regards to a commission. I personally like to know a little about a character, particularly their personality and maybe some key points about them, because I love to incorporate that into a design or piece! If they're confident, they're going to hold themself differently to someone who is shy - if they're an artist, maybe they have paint splotches on their clothing. Concise character traits are ideal.
What I don't need is a small novel explaining their backstory, and even more so it doesn't help me if all this information is randomly scattered across paragraphs of text. I have and will charge people more when they approach me like this - and that might seem harsh, but it's because I have to spend more time and effort, more WORK, to scan through and find what is relevant and organise the information myself into something I can use, as well as identifying what information is missing. If you really want to include lots of backstory, please at least organise the information so I can immediately find a physical description without making me work for it.
One of the best commissioners I've ever had put all of the information and visual references clearly laid out in a single google doc. You wouldn't believe how much time this saves me.
4) Last note. If you are commissioning someone to design a character, using images of other characters or people as reference is perfect! But please, please explain what each image is for, and any information that isn't conveyed by the images. If you have a photo of a person and you like their body type specifically, then say that specifically. If you like the clothes of a character, but not the colours, be sure to say so. I had a situation once before where someone gave me 4 references of white women and said "please make her look like this" and I did exactly as asked - but when I was finished, they asked me to change it: "The character should have a dark skin tone". I can't know these things if you don't tell me! If you communicate clearly what you want, you're more likely to get what you want :)
I hope none of this comes off as too complain-y, but rather as honest advice - artists WILL like working with you a lot more if you make their lives easier, and you'll get better pieces of art back, and faster too! Thanks~