I've been working as an editor for quite a few years by now. Began as an assistant in the late 90's, so I'd say almost ten years.
During these years I've encountered lots of people who wanted to break into the comic book biz. Either as artists or writers. And from that experience I thought I sholuld share some Do's and don'ts for those of you who are serious about getting a freelance job, working with characters someone else has created.
If you are rejected don't tell the editor you know the characters better than he or she. There's most likely a reason why you got rejected. Instead of telling the editor that he/she is a jerk, please ask why your stuff was rejected. But if you sent in a serious submission you most likely got a serious reply with a reason for the rejection.
If you submit new samples after having been rejected once, please make sure that there's a noticeable difference between the first stuff you submitted and the stuff you are sending this time. Trust me that there are too many beeing rejected who says "I'll get back with some better stuff soon." And then they get back within a few days with the same crappy stuff.
Don't become the negative freelancer who always tells the editor how bad the comic book is, how bad the colors are and how poorly written the scripts are. Unless you can say something relevant. The editor is always happy to get feedback. But ... if you only say bad things about the book you are working for the editor might get tired of you. Don't hesitate to tell the editor if something is wrong but at the same time give positive feedback if something is good.
(This naturally goes for the editor too not only the freelancer.)
Turn in your work on time and don't miss a deadline. If you do you might cause a lot of trouble. (Think of all the people who is depending on you, layout staff, printers, distributors etc. ) If you know that you can't make a deadline don't take the assignment. If you are working on an assignment and suddenly discover that you're not gonna make it: Let the editor know ASAP. There's usually one way or another to solve it. Maybe he/she can switch contents with another issue or ask the printer for a week extra. The worst thing you can do is knowing you're gonna be late and not telling the editor.
If an editor calls you with something that needs to be done in a hurry and you take the job and deliver on time, you will not only saving the editors day but he/she will most likely remember that. Anything you can do to make the life of a stressed editor easier will be more apprecciated that you might think at first. If there's an easy job coming up he/she might give it to you just because saved his/her neck that time. That goes for when times get rough also. If you are someone that the editor can trust and rely on, you're more likely to get a new assignment when the budget is tight.
Gotta go for now but I'll try to continue on this list soon.