Peter Mayhew, the actor who played Chewbacca in Star Wars, owns a copy of an early script for the movie that eventually became Episode IV. At the time, it went by the title The Adventures of Luke Starkiller. He's tweeting photos of the pages of the script and promises that he's got a big announcement coming up.
The script includes an early vision of R2-D2 that is quite diferent from his eventual form:
Artoo is a short claw-armed tripod. His face is a mass of computer lights, surrounding a radar eye.
A tripod Artoo might have had an easier time with stairs.