I know there are a lot of good opinions posted so far.

Sorry to add to the overwhelm:

Doing a comic in one day is not a bad goal to have. As my projects tend to grow faster than I can render them it makes sense to scale the project within a time frame one can afford.

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There's my example. Those strips were taking me around a week to write, sketch, layout, choreograph, light, render, and composite. Five days was a good turn around when I could do it.

What makes it go faster?

First subtract, writing, sketching and layout from the timing. You have to do them but for the moment consider pre-process.

With those items considered as finished, you're ready to choreograph. 
Start with the camera in your chosen setting. Render.
Next, in an empty scene bring in your characters using the rendered setting pics. This will allow you to pose your character(s) without the weight of the environment slowing down your computer.
Also if you can, pose the character wearing shoes only. Check your surroundings first to consider whether this is appropriate. This will also keep your Poser-time responsive while you do the gross positioning. Eventually you'll add the clothes, as you'll need to check for poke-through.

Save camera files from the scenery files and load them in your character scenes if it helps speed up the process. If your characters are close to the camera you can sometimes wing it.

A lots been said about the lighting. Very important as you and others have mentioned.

As someone else mentioned, DO POSTWORK. I'm not yelling, just emphasizing.  : )

Your goal is a comic in one day, not to prove how you were able to do dirt being kicked in the air in Poser.  Many pride themselves on what they can do with no postwork. As a static comic that isn't animating that pride will not let you achieve your goal on time, imo.

If you do look at my link, you'll see that I did a lot of my characters in their setting and many times I didn't. Hopefully you are more interested in the story than trying to figure out when I did and didn't. 

What slows things down?

Locations. Locations. Locations. ; ) 
The more places I added, the more lighting I had to do, add props, do more post. If you can minimize your backgrounds then more time can be spent on what's going on in the foreground.

And that's my advice, in one day. ; )
Good Luck