There are definitely threads of similarities runnign through all the films. Like someone said, a "reference". a cliche? I think not. After all this links the SAGA together.
One of the funniest is the long drawn out "NOOOOOOOOO!!!!!" in every film by one of the characters.
Episode I : Obi Wan when Qui-Gon gets run through by Darth Maul
Episode II : Anakin while slaughtering the Sandpeople(Yoda hears it through the Force)
Episode III : When Darth Vader learns he killed Padme, which he did in a way.
Episode IV: Luke when Obi Wan gets sliced in half by Vader.
Episode V: Luke when Vader tells him that he is his father.
Episode VI : When Vader threatens to turn Leia to the darkside and Luke begins his frenzied attack.
One question: Has Lucas always put the numerical sequence 1138 in all the films in reference to his first film THX-1138? I know it's in Episode IV. Luke, disguised as Stormtrooper, tells the Imperial office that he is transferring Chewbacca from Cell Block 1138. Any others?