By default, the reference field lookups will show the Display Value of the referenced record. I would start by looking at how those referenced tables are defined and which fields are set as the Display Value.

Second, on the table where the reference fields are defined (we'll call that the source), make sure the field attributes are referring to the fields on the referenced table (the target table) and not the local source reference field.

For example, if I update the Assigned to (assigned_to) field on the task table, I can add the attributes below to add the email address to the dropdown view:
ref_auto_completer=AJAXTableCompleter,ref_ac_columns_search=true,ref_ac_columns=name;email

Then when I expand the field on the task form, it looks like this: