custom changes

I would like to make some custom changes to an rpm (fetchmail 6.2.5) but
I'm having a little trouble figuring out the steps I need to accomplish
my task:

Here is what I am trying to do:

- use the fetchmail.src.rpm to get the source, patches & spec file.
- modify the source of fetchmail to add some things for me.
- make a new binary and source package.

Here is what happens:

When I use the rpmbuild -ba command and point at the spec file, all
works well. It will build binary and source package fine.

So then I try, rpmbuild -bp command to put the source in the BUILD dir
and start making my changes.  But before I make any I issue
a ./configure...thinking it should configure without error.

But it doesn't.  I'm not worried so much about the error itself as the
fact that the configure doesn't work when used in this way, but it is
clear during the rpmbuild -ba that configure is definitely run and
works.

What do I need to start reading to figure this out?

Mike B.