That's why its basically all the same guy. Superman/Kal-El/Clark Kent. There is no mask unless we're talking the public persona of Clark Kent. That is an act, to the degree that the individual writer wants to accentuate it. But there's never an act when Clark's in private, and there's no acting whatsoever when he's Superman.
That's why there's more truth to the "Clark is the mask" line. It in of itself is not quite honest and marginalizes a more deep dichotomy, but its far truer than saying Superman is a mask. The Clark identity is negotiable, again, it all depends on how deep one wants to go with the act he puts on when in public. But he has to, to some degree. He doesn't have to act at all as Superman and he by no means ever should be characterized as doing so. When he's wearing the costume he's himself. As himself as he'd be able to be just in street clothes as Clark Kent if he had not a care in the world. There is no acting, no theatrics going on there. I'll always will believe that to characterize him in that way to any degree is to have the most basic of misunderstandings of the character. If I had my way no writer who thought that way would be able to get their pen anywhere near the character ever again.