It looked like the show was originally set up with Edwards as the lead and Mark Greene as the leading character but it was Clooney that ended up with the big Hell Or High Water (https://tv.avclub.com/er-s-daring-storm-rescue-made-george-clooney-a-star-1798243061) episode (https://www.tvguide.com/news/er-hell-and-high-water/) that was shot like a film.

And did anybody else think that it was odd that Clooney agreed to return to reunite Doug and Carol and then returned again when ER was near the series finale but he didn't return for Mark Greene's funeral. He stated that he didn't want to take the spotlight away from the Greene character - but that sounded like a silly excuse because Ross and Greene were best friends so it looked lame to not see him at Mark's funeral. There must have been another reason for it.