You need to realize that the word “characters” covers a lot of evils. Not only does it cover the main characters (heroes and villains), but it also covers their respective minions, as well as a number of plot devices and window dressing. It also covers a lot of other things as well. Let's take a long look at characters, and what they really are, and let's ignore the obvious ones.
“Window dressing” characters are your basic extras. They're necessary because they help make your world feel inhabited. A lot of comics forget about them, either because they're too complicated to draw, and so figure out ways to not have to draw them, such as keeping the strip inside or in isolated places. Admittedly, this is more of an artistic consideration than a writing one, but it's something that needs to be considered. After all, it's sorta like going into a forest and not hearing anything; it's sort of creepy.
It may seem weird to think of a “plot device” as a character, but it can happen. Consider Galactus from Marvel Comics; any time that The World Devourer shows up, it's for an important reason. He has put Phoenix on the right path, gotten the Fantastic Four in trouble with the Galactic Council, and ended wars. In short, his appearance moves the plot forward, or helps it conclude. Admittedly, this applies to most of Marvel's cosmic entities, but it applies to any character with a large amount of power that needs to stay in the shadows for most of the comic.
And then there are those characters that aren't characters. The environment itself can be a character; horror comics and any comic set on the ocean would do well to remember that. It may sound weird, but the setting can have its own personality, and that personality will come through; it can be pleasant most of the time, but there are times when it will be a challenge.
The same applies to vehicles and weapons. And I'm not talking intelligent swords or vehicles that operate themselves. Consider Bonaparte from Tank Dominion or Mr. T's van; when those vehicles were in trouble, their drivers were in a state of distress, just as if a son or daughter were in the hospital. When they perform beyond expectations, their drivers are ecstatic. They also encourage them, love them, and basically treat them as their kids. THAT makes them characters, and just as important to the script as the characters. Consider the care some characters give their weapons; those weapons are characters as well. When you say to yourself, “Man! I liked how Excalibur cut through that guy!”, you are no longer treating a sharp, pointy as a mere object, but as one of the cast.
Characters are not necessarily people that interact with the environment. Keep that in mind over the next several postings...