Not realy clear what you want to do... .
but based on your question there is few possible solutions:
1. you want some default value for the field... in that case:
_columns = { 'field_name':fields.text('Field_label') } #define field in your model..
_defaults = { 'field_name' : 'Some default field value' } # will insert that default value upon creting new record in your model..
or,
2. for defined field you wand to show a hint what kind of value you expect in that field , Ž
but no default value is entered... then.. _columns is the same,
and in xml defining view for that field put:
<field name='field_name' placeholder="Some hint text' />
<!-- this will show hint in field, but that is greyed out, and it is only visible until some value is entered in field -->
hope it helps,
may the source be with you : )