From a speech in 1981:
Comic books were my salvation. I read all of them I could get my hands on, and my reading got better and better, and my teachers soon began to marvel that I read with such “expression” while the rest . . . of . . . my . . . class. . . read . . . like . . . this. I could have told them the reason. You need a lot more expression for, “Aha, Superman, now my red kryntonite will turn you into a BOILED EGG!!!” than you do for, “See Spot. See Spot run. Run, Spot, run.”
The link goes to the excerpt on comics, but his full (slightly rambly, but understandably so) speech is here and includes his source of inspiration for Game of Thrones.