For better or worse, we always know what the characters in Sentimental Education look like. Let's take a look at just a few examples:
Hmmm… which of these things is not like the other? That's right—Deslauriers, the lower-class characters is the one who doesn't look all that snazzy. Appearances can tell us a lot about social status, and that fact doesn't go over Frederick's head either. After all, in order to appear handsome and well-off to the Arnouxes, he is willing to blow off his good friend just based on how he looks.
P.S. There is one thing that matters more to Frederick than looking good: Madame Arnoux. Check out this deal with the devil: "Frederick would have been glad to become deaf, infirm, and ugly if, instead, he had an illustrious name and white hair—in short, if he only happened to possess something which would install him in such intimate association with her" (1.4.267). Now that's saying something.
And by action, we mean total action and total inaction. The characters in Sentimental Education can really be separated according to how much action they take—or rather, how much they act upon what they want.
We know for sure that Frederick isn't exactly a go-getter. He lets the revolution just float past him like a television set on in another room, and it takes him twenty years to score with Madame Arnoux—even then, he flubs it up.
So does anyone do anything in this novel? Plenty of people would say no, but there are two characters who break the mold: Sénécal (whether you like him or not) and Dussardier. They don't function solely on personal interest, and they actually act on their convictions. Other than these two, though, it seems to be more about dreams and ideals than actual agency.