Suggestion for how to auto populate field on custom object after a lookup field selection and before saving the record?

I need to auto populate a field on a custom object after selecting a lookup field value and before saving the record. It's the exact same functionality seen on a new contact after selecting the related Account and the Account address auto populates the Contact address before saving. I've tried workflow field update and a trigger which both work fine AFTER saving the record. But I can't figure out how to make it work BEFORE saving the record. Thanks!