Sometimes they ask for special processing as well - which is a security clearance and takes around 3 months for a decision.
If this is the case then all the fees you have paid before you get an interview become irrelevant as the paperwork they relate to expires after so many months
. You have to pay them again when your security clearance comes through.
So probably worth phoning the embassy before doing anything - explain your intentions and ask for a preliminary assessment to assess whether special processing would be required then at least you would know
before spending any money in processing fees.