If you are a person responsible for withholding, accounting for, or depositing or paying specified taxes including NRA withholding and employment taxes, and willfully fail to do so, you can be held personally liable for a penalty equal to the full amount of the unpaid trust fund tax, plus interest. A responsible person for this purpose can be an officer of a corporation, a partner, a sole proprietor, or an employee of any form of business. A trustee or agent with authority over the funds of the business can also be held responsible for the penalty.
"Willfully" in this case means voluntarily, consciously, and intentionally. You are acting willfully if you pay other expenses of the business instead of the withholding taxes.
For more information refer to Employment Taxes and the Trust Fund Recovery Penalty (TFRP)