I think a lot of the problem is the writers and directors didn't really know how to handle a character who was invulnerable.

Many times, especially in Season One, Superman is shoehorned into the last four or five minutes of an otherwise realistic, film noir-ish story, and once he shows up, credibility goes out the window.

Because it would be visually boring to just have him stand around and let objects passively bounce off him, he interacts with the environment and characters like a normal guy, albeit one in pajamas. So he ducks, weaves, runs, and fistfights.

Really, if it weren't for the bouncing bullets, a person unfamiliar with the character might gather from watching the show that Superman's only powers are flight and the ability to look through walls (and see things from a perspective that would be impossible, most times, based on where he is standing when he uses the x-ray vision).

Still great fun, though, and Reeves always plays it straight, even when ducking from the gun!