This is why the original actor for Marty, the very talented Eric Stoltz, was fired from the role. Stoltz is a great actor, but Stoltz could not make Marty work because there was nothing on the page for Stoltz to play and he didn't have Fox's gift for putting his own personality into otherwise characterless characters.
With the director being the co-writer, it was especially bad that Stoltz wasn't given enough encouragement. In a 1994 article for the L.A. Times, Stoltz said: "Zemeckis told me I was giving a good performance in a film he didn’t want to make – contemplative and thoughtful instead of comedic. I felt I could have done the part had he pointed me in that direction."
As for the other casting finalists, Ralph Macchio would have looked more poignant if not as intense as Stoltz. John Cusack would have looked too sleepy or indifferent whereas C. Thomas Howell might have come off as unwholesome.