What's the difference between writing a graphic novel and writing a comic book?
The specifics? I'm glad you asked.
For starters, the most obvious difference is that a graphic novel is longer, but that just doesn't mean it takes up more pages. It also means it takes up more plotting. It takes up more thought for building up your characters. It takes up more time to let your story unfold in a way that it might not be able to in a monthly comics that needs it's own "3 bangs and a cliffhanger" each month. (Even if it's being collected later as a trade, because a trade collection is a different animal than a graphic novel, though the two are often marketed under the same name.)
When writing a graphic novel, you must think about it the same way a writer thinks of writing a novel. A monthly comic is akin to a serialized group of short stories and must meet those criteria, but a graphic novel is far more reaching than that. Sub-plots, minor characters, build-up scenes, segues, denouement, etc. are all going to demand your attention in a graphic novel, and you'll have the time and the room to play with them -- providing their the best tools to use in your story.
My favorite part of crafting a longer-form work though is that I don't have to follow the arbitrary "22 pages ending on a cliffhanger" rule. In an original graphic novel, if I need a 3-page chapter, that's fine. I'll add it. If a chapter needs to go into 28 pages, that's not a problem either. Because the book isn't designed to be read monthly. It's a take a bite at a time to devour the elephant kind of experience instead.