Two questions re grid/details view and dropdownlists-newbie

[et]

I have a drop down list in a template field in a detailsview for when I add
or enter a record, the remaining fields are bound fields. Simple enough.
This drop down list contains the name of a product. I want to be able to
populate some of the bound fields automatically when a user selects the
product from the drop down list while in add or edit mode, and I'm not even
sure where to start, how do I programmatically grab a bound field and
populate it, still allowing the user to change it before they save the
record. Thanks for your help.