NSC Interest is taxable. But under sec 80 C if interest in reinvested then you can claim deduction under this section. Usually NSC is for 6 years. You can't withdraw interest in mid. You will receive it only on maturity.
So answer for your question.
1. Whether NSC interest is taxable?
Yes. But deduction can be claimed under section 80 C.
2. What about withdrawn interest?
You can't withdraw interest in middle. You will withdraw it on maturity. Full interest is not taxable. Only last year interest is taxable. Because previous years interests, you would have already offered as income. So finally to conclude 6th year interest which is not reinvest is alone taxed on the year of receipt.
Every year you have to add the accrued interest to you income and deduct it under 80C,