If the characters engage in more or less random actions, all results from those actions will also be more or less random. If the script then acts as divine intervention, the interest of the characters will diminish because they aren't actually doing anything, stuff just happens. If the characters aren't truly agents, then only their personal charm will attract viewers (which we can already tell is going to just mean people who like Noah Wyle and/or Colin Cunningham.)
In the long run, the plotting must address the problems inherent in why some alien race would conduct such a nutty invasion when there is no conceivable benefit (or alternatively, if the invaders are primitive enough they could benefit, how can they even travel between stars.) If it doesn't, there's just us vs. them, in a necessarily implausible scenario, which brings us back again, do we invest in these characters enough to enjoy seeing the script hand them a victory? (And the answer is still, only if you like Noah Wyle and/or Colin Cunningham.)
If the plotting is adroit enough, the characters would reasonably make different choices about what to do, which is the essence of drama. The high stakes (survival of humanity itself) would ramp up the tension.
If the plotting fails to give a deeper kind of faux-realism, where the space invasion made sense, they would just be villains for the sake of being bad. The conflict between them and us would not really be much more meaningful than a match between two boxers, except one's really ugly and no one likes ugly. You could go the comedy route like SG-1, except that it's pretty obvious these guys aren't funny.
And there aren't even any Daniel Jackson characters who've got a very different perspective and values. Superficially, Pope is different but Pope is a total cartoon. Cartoons don't fit in such solemn shows as Falling Skies.
