Re: Christopher Walken as Han:
If Walken had been cast it wouldn't have been Mark Hamill as Luke. Lucas wanted to cast Han and Luke as a duo--he thought Hamill was the actor who worked best opposite Ford. But to play against Walken, Lucas had a different unknown in mind, a guy named Will Seltzer, whom Lucas saw as a bit more mature and "grown-up" than the golly-gee-gosh wide-eyed Hamill.
Lucas compared Seltzer's take on Luke to Richard Dreyfuss in American Graffiti--in fact, when Dreyfuss refused to come back for More American Graffiti, Seltzer was cast in the film essentially as his replacement. Seltzer hasn't been in much of anything since. (Although, for Nintendo nerds, he's probably best known as the villain Putnam in 1989's glorified advertisement The Wizard.)
In his audition tapes Seltzer looks rather like a young Jesse Eisenberg--curly brown hair and gray eyes. I would guess Lucas deep down wanted somebody more WASP-looking, more the blond Flash Gordon type, for Luke; that's what all of Ralph McQuarrie and John Mollo's concept art shows.
As for Leia, apparently the other lead contender for the role (besides Fisher) was Terri Nunn, who went on to fame as lead singer of the band Berlin. Nunn was a minor at the time--apropos of the script, where Leia is described as 16 to Luke's 20 (NOT twins yet!). But Lucas went with Fisher (who was 18) largely to avoid the legal issues involved with having an underage cast member.
FWIW, apparently Hamill and Fisher themselves thought the most likely contenders to beat them in the casting call were Robby Benson and Jodie Foster. Both were actually considered--Foster, like Nunn, was a minor at the time.
More far-out casting ideas Lucas considered, early on in casting sessions, were to cast an African-American as Han (possibly inspired by Leigh Brackett's black space outlaw Eric John Stark?) and a Japanese-American actress as Leia (along with Toshiro Mifune as Ben Kenobi--the two characters coming straight out of The Hidden Fortress). Lucas actually selected actor Glynn Turman as his candidate for a black Han; this impulse later gave birth to the character of Lando, who when you get right down to it is really a clone of Han.