I am not too sure whether you mean the spending as in how you ran up the debts or spending as in your monthly expenditure.
During the interview the OR will want to know how you managed to get into debt however will not judge you. He will also go through your monthly expenditure that you detail in the 6.28 form and may change the amounts if he deems them to be above the recommended guidelines.
After the interview he will send you the monthly expenditure amount along with the amount of IPA you will make. You can dispute the monthly expenditure if you don't agree with it.
You need to inform the OR of any changes to your income following the interview and he is permitted to contact you at any time before the BR is discharged.