S Corp Tax Form 1120S Line 8

It should be the gross amount paid or accrued or otherwise considered paid or accrued to the employees. The cost incurred, if you'll go for that. And it depends on what basis of accounting the corporation used in preparing its income tax returns.

And I'm pretty sure the amount that should go on that line will [ought to] include amounts that are considered to have been "withheld" from employees' paychecks, even though those amounts haven't been paid yet by the employer (i.e., they're still liabilities...).

The gross amount of wages should go on line 8 with an exception for officers, whose wages should go on line 7. There are also some situations where wages would be included in cost of goods sold, which goes on line 2. Elective contributions to a 401(k) plan, deferred payment arrangements or a salary reduction SEP can also complicate matters.

The very fact that you asked such a question leads me to respectfully suggest that you contact an accountant to prepare the 1120S and any relevant state returns.