Great movie. Fortunately, I saw this movie before I became a guitarist and I remember thinking it looked just like Ralph Machio was playing those parts. Naturally, it doesn't look like it now but the fundamental motions he's doing match what the ear hears: Ascending runs, he's going up the neck/strings. Descending runs, he's doing down the neck/strings. Stagnant licks, such as the begining of Eugene's Trick Bag, he was staying right in place when he needed to. Personally, I think they achieved a LOT with a guy playing a guitar role who didn't play.
Everything you hear at the end when Steve Vai enters the picture is a product of Vai's compositions. Listen closely and you can hear where Vai is actually playing the Ralph Machio final solo in places with his fingers instead of a pick, just like they show at certain times when they pan over to Ralph's hand.
In being able to vividly recall what I thought when I first saw the movie before picking up a guitar and then analyzing the basic fundamentals of when to have Ralph ascend/descend, regardless of how wrong it may literally be, I think they achieved EXACTLY what they were going for and did an excellent job doing so.
BTW, I'm glad I didn't click this and have you referencing the Britney Spear's movie Crossroads.