meaning & use of "IT"

What the second 'it' does for me is to make who a bit more important than the creeping about. Without the second 'it', the attention turns more toward the creeping about.

The second "it" is correctly used, but in a construction less often seen than others.
The understanding of it may be clearer if you take "who it was" as a phrase group, pause, then continue with "creeping about". The last example below shows this.

I couldn't tell who was creeping about.
Someone was creeping about. Who was it?
Who was it who was creeping about?

So if you were to use a comma where would you put it.

a) It was dark out there and I couldn't really tell who it was, creeping about.

b) It was dark out there, and I couldn't really tell who it was creeping about.

Many thanks,

Hela

I think you're asking CJ, and he can answer for himself.

I don't think commas are called for. Let me try it one more way. Without the second 'it', 'who was creeping about' seems very generalized and uncountable. 'Who', without the 'it', could mean a single individual, a group of individuals, or even encompass 'what was creeping about', such as what wild animal(s), ghost(s), insect(s).

Thanks, davkett, it's much clearer now.

All the best,

Hela

If I were to use a comma, it would only be to help you understand the structure better, and in that case I would use a). However, acting as an editor I would remove any commas in the sentence as there is nothing there that requires a comma. (I agree with davkett on this point.)

CJ