Just imagine how he could mould Vader into a believably dark character......
"Luke, you son of a bitch, join me and we'll fuck the emperor together....."
"Don't underestimate the dark fucking side. You know what will happen if you understimate it."
"Are you lookin' at me? I know you're fuckin' lookin' at me. I can sense it you dumb bastard."
"Listen Bobba, if the emperor's prize is damaged in anyway, Jabba's gonna be digging your good-for-nothin' body outta the Mos Eisley parking lot!"
"He will join us master, or I'll get Darth "Joe" Pesci to knee-cap the son-of-a-bitch."