Well i was on my warlock in a BG yesterday and having a felhunter with paranoia obviously lets you see rouges from a pretty good distance, so i targeted him and began to cast shadow bolt, right as i saw him about to sap me, there was no cast bar but 4-5 seconds later when he ambushed me i instant casted shadowbolt. Ive tried this over and over and it seems to work 40% of the time you just need to begin casting at the perfect moment.
edit: yea im stupid i meant felhunter :P