If a character says something, the lips should be open. Unless it’s “hmm” or “mmm” or something equally audible with closed lips, they should be open, just like when you talk. Since they are comics, you can also use this not only as expression “this one is talking” but depending on how it’s made, also of how the character is talking. Like thin opening says usually a light voice, or quiet. If its only a corner open, they say it to someone on the side, not meant to be heard to everyone else. Mouth wide open is yelling. Please don’t just open the mouths wide all the time, I have enough yelling going on already.

At the same time the talking also can be enhanced for mood. Sad or happy face saying, or yelling something look different.

Since my number one gripe is proportions, this also plays an effect. Our brains are wired for pattern recognition (proportion is a pattern). So we recognize patterns on faces. When someone is smiling, but their eyes don’t lift, they are either not genuine (the creepy sales rep face) or they have some botox in the wrong places. But if the whole face smiles, we suddenly feel the difference. So if the mouth is neutral but the rest is “happy” this just looks wrong.

The main concern though should be consistency. If your characters always talk with mouths open, keep it that way, don’t mix and match or your readers get that wrong feeling again.

An exception exists of course, which means you can use it as a style tool for example if someone has an extremely large or stupid grin on their face (the chibi smiles in anime for example). But use them sparingly and for exact effects.

The visual is a (pun intended) no brainer. Look, aha, done. In a comic this means you take a look and you know how the character in general looks. Of course you can do a lot more funky things, show poses, costumes and so on, but the reader is not really imagining how that hero looks like. The reader knows. Same goes for TV and movies of course.

In the cerebral way, there is no longer a “right way” the character looks like. There is a description in text (or by audio) and that’s it. From that and the persons own visual memories, everyone crafts their own image. That might match in many cases, even with the authors image, but there are always a lot of differences.

This is why I like people drawing characters I created in a written story. What they draw is what I imagined, tried to describe on paper and then they re-imagined in their own head after reading those words. Such a result might be horrifying to witness as it often is completely not what the author has imagined, but that’s not really a bad thing. The reader makes the story their own by reading and imagining it, which is one of the best ways to know they like it because they actually enjoy the story enough to get creative with it.

And of course there is a time thing. I watched recently someone live stream how he paints with a tablet, from scratch. Watching paint dry is more amusing. In that same time I could write and edit an entire chapter with half a dozen new characters. This is not “writing is better than comics”, I have been doing both for a long while now, so I know what I’m talking about here and not taking sides. Just pointing out the discrepancy.

If you want the people to have an exact image, you need to go visual (comic, video). You pay for it with time of course (and other money costing things like equipment), while you can get faster (and cheaper) results by just writing.