Im starting with this iptables stuff and trying to manage a home network trough linux and adsl + pppoe

I have two simple questions.

I have my adsl router conected to my eth1

When im setting my iptables rules, should I think that eth0 routes packages through eth1 to ppp0 ?

Because, if that is, my drop rules should be defined on eth1, and my input ones on eth0

Our should i think that eth1 does not matter and my reliable is eth0 and my unreliable ppp0 ?

Please forgive my crude english

Thanx