More to the point, whether the character chooses the path of direct action or the path of thoughtful action. If a character chooses thoughtful action, he's likely a character who sides with reason and strategy. On the other hand, a character who chooses direct action is likely prone to violence over contemplation.
A rule of thumb when reading Asimov: a character who acts like a scientist will win against a character who acts like a warrior. It is science fiction after all. Here's what we're talking about:
Leopold and Wienis are men of direct action. As Leopold says to Wienis, "You're absolutely right, you know. We must strike [the Foundation] first. It's simply self-defense" (III.3.49).
Opposite those guys is Hardin. He's thinks that "at each crisis [the Foundation's] freedom of action would become circumscribed to the point where only one course of action [was] possible" (III.2.68). He need only wait for that moment to come. When it does, he soundly defeats Wienis and Leopold without issuing a single order.
Most stories that use props as a characterization tool tend to use a very specific prop to characterize their characters. If you've ever read Catcher in the Rye, then you'd notice Holden Caulfield's red hunting hat is one such prop.
But in Foundation, we've just technology. If a character holds a piece of tech, pay attention.
How about an example? When Hardin meets Anselm haut Rodric, he compliments Rodric in a ceremony involving a "blaster specifically borrowed for the occasion" (II.2.2). The fact that Hardin has to borrow the blaster tells us something about him; he's a pacifist, and the way Hardin solves the Foundation's first crisis proves this fact.
On the other hand, Rodric came with his own blaster and later "monopoliz[es] the conversation by describing […] his own exploits as battalion head during the recent war" (II.2.22). Rodric, it seems, is a bit of a warmonger.
Speech and Dialogue
You want to get all the best lines in an Asimov novel? Then you'd better be a thinker. Smart folks get witty dialogue all throughout Foundation, while those who use talents other than their brains to get by—well, they come off looking not too smart.
The trial provides a perfect example:
Q. Let us see, Dr. Seldon. How many men are now engaged in the project of which you are head?
A. Fifty mathematicians.
Q. Including Dr. Gaal Dornick?
A. Dr. Dornick is the fifty-first.
Q. Oh, we have fifty-one then? Search your memory, Dr. Seldon. Perhaps there are fifty-two or fifty-three? Or perhaps even more?
A. Dr. Dornick has not yet formally joined my organization. When he does, the membership will be fifty-one. It is now fifty, as I have said. (I.6.6-11)
It goes on like this. Every time the Commission's Advocate tries to corner Seldon in a logical snare, Seldon breaks free and turns it on him. Later, we'll see Hardin and Mallow doing the same thing.