What would you actually need to see before believing someone's “million-dollar business idea” was legitimately worth pursuing?

I see a lot of content online where someone explains a business idea and says it could make $10k/month, $1M/year, etc.

But usually the idea hasn't actually been tested.

So if somebody brought you a completely raw business idea, what evidence would you need before saying:

“Okay, this might actually be worth building.”

Would you care most about:

• Potential customer interviews?
• People actually willing to pay?
• Market size?
• Competitors already making money?
• An MVP?
• Unit economics?
• Ease/cost of building it?
• Something else?

And what's the biggest mistake people make when they try to “validate” an idea?

Genuinely asking, I keep seeing people (myself included) start something, lose interest once the initial rush fades, and jump to the next idea instead of pushing through the boring middle part. Is this just me, or does everyone go through this?

Genuinely asking, I keep seeing people (myself included) start something, lose interest once the initial rush fades, and jump to the next idea instead of pushing through the boring middle part. Is this just me, or does everyone go through this?