I always liked this explanation of Kirby vs. Ditko pencils on ASM:
"Lee and Kirby "immediately sat down for a story conference" and Lee afterward directed Kirby to flesh out the character and draw some pages. "A day or two later", Kirby showed Lee the first six pages, and, as Lee recalled, "I hated the way he was doing it. Not that he did it badly — it just wasn't the character I wanted; it was too heroic".
Lee turned to Ditko, who developed a visual motif Lee found satisfactory,[21] although Lee would later replace Ditko's original cover with one penciled by Kirby. Ditko said, "The Spider-Man pages Stan showed me were nothing like the (eventually) published character. In fact, the only drawings of Spider-Man were on the splash [i.e., page 1] and at the end [where] Kirby had the guy leaping at you with a web gun... Anyway, the first five pages took place in the home, and the kid finds a ring and turns into Spider-Man."
Ditko also recalled that, "One of the first things I did was to work up a costume. A vital, visual part of the character. I had to know how he looked ... before I did any breakdowns. For example: A clinging power so he wouldn't have hard shoes or boots, a hidden wrist-shooter versus a web gun and holster, etc. ... I wasn't sure Stan would like the idea of covering the character's face but I did it because it hid an obviously boyish face. It would also add mystery to the character...."