Greying out or locking certain columns
 

I am trying to construct a fairly simple database, but I have a column with a
drop-down menu and I want subsequent columns to only be applicable when
certain options are chosen.

E.g. if the drop-down menu gave the options "apple", "banana", "orange",
"pear" then I would want certain data to be entered in later columns if the
option "apple" was picked, but would want other columns filled in if "pear"
was picked.

Is there a way to lock or grey-out certain columns in Excel, so that they
become unlocked or active only when certain options from a drop-down menu are
picked? Or is there another way to make sure that someone only enters data in
relevant fields and not others?