At first, Spike (Freddie) and Buffy (Sam) didn't start off well, but eventually turned friends/allies.

They often fought a lot.

When Spike met Buffy, he was in love with Drusilla (Carly)

Spike was under the false impression Buffy was flirting with him, while it was really Faith in Buffy's body - and Freddie was under the false impression Sam was flirting with him, while it was really Melanie.

Spike secretly loved Buffy, even when with Drusilla and Harmony.

In the final episode, Buffy finally admitted that she loved Spike - meaning she loved him all along.

This signifies Spike and Buffy secretly loved each other, and first hid their relationship from others. Similar to how Freddie and Sam are [possibly] secretly in love with each other, and hid their kiss from others.

Winry (Sam) often throws a wrench at Edward's (Freddie's) head when she is frustrated with him.

The two are actually close friends, no matter how much they argue.

Both have feelings for each other but don't realize it/they hide their feelings.

Winry realized she loved Ed before he realized he loved her.

Their fights are sometimes resolved by Alphonse (Carly).

Winry is very passionate about auto mail and knows a lot about it, like Freddie is with technology. Edward often makes fun of her for this, like Sam does with Freddie. Ed knows a lot about alchemy, and Winry does the same thing to him.

They have nicknames (pet names) for each other. Ed calls Winry "Gearhead" and she calls him "Alchemy Freak."