1. How did he get his scar
In Legends, Anakin gains the scar from a duel, with Asajj Ventress. This one in one of the Clone War comics published by Dark Horse. In canon, Anakin obtained the scar in the empty space between Attack of the Clones and The Clone Wars TV show (the show starts about a year in the war).
2. The Actors
Darth Vader was played by six different actors throughout the films. David Prowse and Bob Anderson played him in Episodes IV-VI. James Earl Jones voiced his character in episodes III-VI. Sebastian Shaw played him in Episode VI. Jake Lloyd in Episode I and Hayden Christensen in Episode II and Episode III.
3. End credit proplom
David Prowse was convinced the film would be a massive flop, and he refused to allow his name in the original credits, as he didn’t want this destroying his career.