When you're playing "backwards" in G on a C box, the notes ABDF are all suck notes. You get a very bluesy sound by smearing from A to B, giving a very wild third to the chord of G, and the facilty of an F natural on the pull gives you a good G7 chord when youre changing to a C chord. There's a lot more too it, but that is the basis of a lot that is distinctive about cajun/zydeco box-playing. The equivalent thing if you have a D/G melodeon is to play tunes in A on the D row.
    A distinctive feature of old-time cajun and creole fiddle players is their readiness to go up the neck a bit (forbidden territory to most Brit/Irish traddies). C# and D on the E-string are quite common ( assuming conventional tuning, that would be B and C in cajun tuning) . If you're competing with a box-player with 4 reeds per note at a noisy dance, you need to get up there now and again to cut through.