If you don't have enough horsepower to render shadows, consider manually adding in some light and shadow and/or depth-of-field effects for those shots with static backgrounds. This will break up the flat look of the visuals without adding rendering time.

To add a more cinematographic look to your animation, don't have your background in focus 100% of the time. There are a number of artistic and practical reasons for this.

Blurring the background (or simulated depth-of-field) is really handy in more ways than just looks. You can use DOF to blur out a visually lacking background. I recall a shot of a woman speaking with the coast as a backdrop but the water doesn't sparkle and it isn't animated so it looks unnatural, like a billboard. You can alleviate that by blurring the background.

DOF can also serve to help focus the action or visually simplify a scene. There's a shot where you see a close-up of two handguns firing against a really busy and distracting brick wall texture. Adding a degree of blur to the background will make that brick wall less intrusive, and it'll help intensify the scene by helping the viewer focus even more on the action.

I noticed that your characters don't blink and don't move their eyes around often enough. It makes them look a little dead. When animating the eyes, it's key to remember that humans make a lot of involuntary eye movements. An example would be how a person's gaze tends to lead a head turn. i.e. 'Eyes lead the direction of motion.' Using an eye target can help you animate this type of action.

Another example would be how a relaxed person blinks or half blinks when doing a rapid head turn. It's an involuntary eye movement. I bet you don't even notice when you're doing it yourself. :-) However, when a person's highly alert or paying specific attention, they don't do it.

Blinking is important too. It's a combination of automatic and involuntary eye movement so it's an action that everyone does. You notice when characters don't do it so it's important to take it into consideration when you're doing character animation.

Regarding character motion, it's good to keep in mind that we tend to move with a mixture of actions (both linear and arcing) of varying speed with varying degrees of accelerations (ease-outs) and decelerations (ease-ins).

So I'd recommend not using Poser's spline interpolation for everything. It adds ease-ins/outs that are too consistent, too smooth. A side effect is that it can slow down the action too much and it can give an unnatural 'flowing' quality to the character movement. People don't move that smoothly and carefully in real life so manually adding and tweaking a combination of spline, linear and constant interpolations with ease-ins/outs is required for better character animation.

It's good to keep in mind that almost all action in real life generally tends to moves in arcs. So character motions with a little arc to them looks more realistic than linear movement, which looks very mechanical. Combined with a bit of ease-in/out, you can get more natural character animation.

Overall, it looks pretty good. It's coming together.