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の話をしてた

was talking about

→ の
phrase Common #4,145
was talking about Casual form of の話をしていた. The の marks what was being discussed, and 話をしていた means “was talking.”
JA: 昨日、友達と映画の話をしてた。
EN: I was talking about movies with my friend yesterday.

あなたの話をしてた

I was talking about you.

— 燃ゆる女の肖像 (2019)

キノコの話をしてたのよ。

I was talking about mushrooms.

お父さんの銃の話をしてたんだ

I was talking about Dad's gun.

— ミスティック・リバー (2003)

何の話をしてたんだっけ？

What were we talking about again?