When Luke, Obi-Wan, C3-P0 and Artoo first enter the docking bay where the Melinnium Falcon is docked, they walk down a flight of stairs. You can see Artoo, who clearly cannot negotiate stairs, behind the others, pivoting around like he's walking down the steps. If you look closely you see that he is floating completely off the ground and twisting to make it look like he's walking down the steps. He's either being pivoted on some kind of stand or hanging by cords. It's pretty funny looking.