??? quick question about voice over ???

I'd start with the action first so that the reader is grounded in what's going on and knows what the voice is talking about.

Yeah, what krano said (the casino script is a good example).

You said 'think advert' - meaning you are writing advertising copy? Typically that is done with an AV format - not a standard screenplay format. This is a dual collum format with the action (usually) on the left in boxes that line up with the dialogue/voiceover on the right.

IF I understood your question correctly?

If you are talking about standard formating in a screenplay, then usually the action will proceed the voice - but the voice can 'lead' the action as well.