In a standard P2PKH configuration, the scripts look like:
Pubkey script: OP_DUP OP_HASH160 <PubKeyHash> OP_EQUALVERIFY OP_CHECKSIG Signature script: <sig> <pubkey>
When redeeming P2PKH outputs, OP_DUP is done because the pubkey is used twice, once in the hash and then verifying the hashes match, and then again in verifying the signature. So, here it seems like OP_HASH160 is consuming the top stack element and replacing it with the HASH160() of what was there.
In P2SH transactions, the scripts look like:
Pubkey script: OP_HASH160 <Hash160(redeemScript)> OP_EQUAL Signature script: <sig> [sig] [sig...] <redeemScript>
When redeeming P2SH outputs, there is no OP_DUP. Won't this mean that the redeemScript will be consumed by the OP_HASH160? How can the redeemScript be executed if it was consumed by the OP_HASH160? I think I am missing something here.