Orpingtons have loose fluffy, somewhat Curly feathers similar to Cochins. Sussex have straighter feathers, laying closer to the body. Looking at cochins, orpingtons, sussex, wyandottes, and leghorns in profile. Cochins have short tails curving fairly closely around their backside. Orpingtons don't curl so closely, but they do in similar shape. Wyandotte tails almost curve up over the back. Leghorns stick out the back angling somewhat upward like the tail of a weather vane. Sussex tails are not as curved as a wyandotte but not as long as a leghorn.
Look at the way the feathers move when they get hit with a breeze. Orpingtons and cochins both get ruffled quickly. Leghorns, aside from the tail almost don't get ruffled. Wyandotte and Sussex fall in between the others.
Both of those things (along with body shape and size) are used to differentiate breeds of chicken.....even when comb, feather pattern and leg color are the same. Unless you're a serious breeder, glancing at different breeds of chicken that look similar (speckled sussex vs Jubilee orpington, Colombian wyandotte vs light Brahma, barred rock vs Cuckoo marans) the differences tend to be "spot the differences, hard mode" sorts of things.