How does payroll interact with HR?

HR will assist with various pension and 401k plans and relate these to payroll.

It depends on the size of the firm. Often HR assists new hires with their W-4 firms and gives to payroll so that these new hires can get paid. Also HR will assist with various pension and 401k plans and relate these to payroll. HR will have new hires complete the I-9 forms and hand over to payroll. And when an employer is fired or payed of information is relayed to payroll by HR.