It depends on situation. Usually you're required to fill in your 'current salary' on the Application Form so interviewers hardly ask you directly for your current pay since it's already stated on d form. Then they will check how much you marked up (from your current pay to expected pay for this firm). They will decide whether it's justifiable or not and perhaps nego if they think you're demanding too high of a price.

Payslip is indeed P&C so you can refuse to show (just give excuse you didn't bring etc) during interviews. But if you're a staff there ady... Im not so sure whether you can refuse the HR/ Accounts from getting hold of it. We need HR experts to clarify this point ^^;