not really or really not?

[raymondaliasapollyon]

Hi,

Should "not really" or "really not" be used in the following?

The saying is used to say that someone seems to be innocent, kind, etc. when they are not really/really not.

I'd appreciate your help.

 

[jutfrank]

Here is a very simple way to show the difference in sense that the different placement of really could make:

really not = very much not
not really = not, in fact

 

[raymondaliasapollyon]

Here is a very simple way to show the difference in sense that the different placement of really could make:

really not = very much not
not really = not, in fact

If "not really" means "not, in fact," should there be a comma in the following?

John seems to be innocent when he is not(,) really.

 

[jutfrank]

I personally think the comma works well there but others might not use one.