how to change lookup field's view based on form data ?

Based on form data (a check box) look up field on form is required to returned different contacts. If Employer check box on form is checked look up field view should return only those contacts (filtered) who belongs to current form's employer other wise look up should return all contacts.

Any idea how to achieve it ? Please guide if multiple ways are available to do that. Will highly appreciate if a relevant link is shared.

Thanks