Just a couple of clarifications:
2 - John Byrne explained that Clark hunches forward a bit, combs his hair back, wear deep-lensed glasses and speaks in a much softer tone than Superman. The result is a completely different visual, like Robert de Niro usually do in his movies. Add to that the fact that no one thinks that Supes has a secret id, and the illusion is cast. during the Byrne run, Supes used a trick that Jay Garrick (Flash I) came up with, and constantly blurred his face to avoid photographs, so no one knew the exact details of his face.