The income share agreement has no interest. It's a flat percentage of your income, which is discharged once one of three things has happened: 1) you've reached your payment cap (the maximum you will pay), 2) you've made 19 monthly payments, or 3) you have deferred payment for 60 months (even if you haven't paid us anything).
If you have to stop work for any reason, your ISA pauses. There is no interest and you do not have to pay again until you earn over your payment threshold. If you are unable to make a payment for 60 months, your obligation is discharged.
We are using different ISA providers for each of our target countries, so the mechanisms of the ISAs are slightly different but follow the above terms in broad strokes.
Please see these country specific terms in your ISA contract.