According to the interview in “Star Wars Insider,” Jackson asked George Lucas indirectly through a media appearance. He said:
“I pretty much campaigned to be in this movie. I knew he was shooting it, and folks ask you a lot who are some of the directors that you haven’t worked with that you want to work with. I generally don’t think about things like that, but I knew he was doing this film and I was doing this interview show in London, so I said, ‘George Lucas. He’s doing some Star Wars movie and I’d really like to be in it.’ Apparently, somebody told him.”
This person was casting director Robin Gurland, who spoke to Lucas about it. Jackson said he was “pretty much willing to be a stormtrooper, just as long as I knew I was in the movie.” He said he didn’t even mind if no one else knew he was there. However, the role was kept a secret even from him until he got to the set. Jackson said, “Once they started fitting Jedi robes on me, I was like, ‘Hey who is this guy? This is gonna be cool.’ So there was no way I was not going to do it.”
This is a level of awesome that has rarely been achieved by anyone, and I’ll die on this hill. So how did he get a purple lightsaber when no one had one before?