Comic production is similar to film production.
A publisher has an idea and hires a writer, or a writer contacts a publisher with an idea. Then the writer is teamed with an artist to create the story. How much choice either have varies, but in the end, the final product is a collaboration between the two. Judge Dredd was an idea by the main writer to this day, John Wagner, but he's credited as co-creator with Carlos Ezquerra, the artist, because he invented what Dredd looks like. Since then, both have worked extensively on Dredd, but not always together. Carlos has drawn for other writers, Wagner has written for other artists (in fact, Judge Dredd: America, my favourite Dredd story, was drawn by Colin MacNeil). They've created other work together, like Strontium Dogs and so far as I'm aware, have been the only people to work on it. Dredd though is probably owned by the publishers more than Dogs, so they are able to get more variety in tone.
In the food chain, the colourist and inkers are like cinematographers, if the artist isn't doing it themselves. They're there to do job, which they may do incredibly well, but it's a production job all the same. Think about the "tracer" jokes in Chasing Amy...
I think Ezquerra inks and colours his own work, but I've heard lots of stories about people not being happy with the result. On regular strips in 2000ad, I understand that the writer and artist are not forced to meet and the general relationship between the two groups is of mutual hatred!
The letterer's are unsung heroes. After all, they literally do the bubbles, but where they go has already been suggested and what they say is in stone. For the most part, I suppose they have the least creative input, although sometimes they are very inventive and the different styles help tell the story.
