All Good Actors are Hams
This is reasonably funny and reasonably true-to-life since the faults the brothers pick on are the kinds of things that brothers might in fact pick on (and reflect both brothers' personalities).
What makes it hilarious is that the actors don't flinch at hamming up their character's faults.
With utter unself-consciousness, Jensen Ackles (as the oaf) stuffs twenty or more nuts into his mouth while Jared Padalecki (as the overly emotional empathizer) hugs a bewildered student for being a "trooper" and "too precious for this world." Of course the whole thing devolves into a fight over the equivalent of car keys (it's a money clip).
The jokes go beyond the actors and the "insider" momentum of Season 2. One doesn't have to know who the characters are (though it helps) to enjoy their physical comedy. For reasons best explored by humor researchers, watching a man overstuff his mouth and then try to talk makes a person laugh. (Really: I haven't laughed this hard since Buffy.)
At this point (I've just starting watching Supernatural), I decided that I not only enjoy the well-plotted (and deceptively simple) episodes: I like the actors as well.
Every good actor should be willing to ham it up. It can't be easy since, as the gentlemanly Leonard Nimoy points out, actors must be willing to protect their characters. But hey, even Spock giggled at the whales (of course, knowing when to be serious and when to giggle is an art in itself; Tom Hanks never seems to make mistakes here--he always picks the best film to showcase his talents--but many actors do).
When it is time for ham, it is time for ham. Shakespeare understood this. In her murder mysteries, Josephine Tey argues that actors should never allow their personal lives to spill over into the performance; it makes the audience uncomfortable. Of course, Tey hadn't seen reality television. But her point is valid. A self-conscious actor stops living in the moment and starts watching him or herself, which shatters the suspension of disbelief; the story--and the audience--flounder. Robert Downey, Jr. may always act Robert Downey, Jr. but he is acting. He is being Sherlock or Tony for the sake of being Sherlock or Tony.
To put it another way, he is having fun! In a discussion of The Desolation of Smaug, Benedict Cumberbatch ruminates on the freedom of motion capture. It might not seem like acting yet the play-like, unself-conscious nature of the exercise can make it more like a true "act" than presenting oneself to the cameras. (Instead, the cameras come to the actor.)
I acted in high school (and a little in college: my English Department put on some plays), and I utilize some of that experience as a teacher, drawing on energy to present a topic for its own sake without thinking about myself. However, I could never quite reach that point of complete submersion (which is why I considered playwriting over playacting).
The best actors make it about the part. Being able to ham it up is the test.