What are the principles of a JTBD interview?
These are directly taken from Bob Moesta's book Demand Side Sales.
They're meant to act as guardrails that safely keep you on the right path during an interview. I'm writing it here so I can prime myself before an interview. If you find it helpful please buy Bob's book!
Humility: You're not smart enough to instinctively know why people really buy.
You need to be an investigator to understand the causal events that led someone to make a purchase. There are too many causes and too many different sequences, which makes it hard to see patterns. You can't sit in a conference room and figure this out.
Causality: Everything is caused; nothing is random.
The notion of randomness was created to help us be "okay" with no explanation. But it's overused and has the side effect of enabling us to waste time and wait.
Tradeoffs: Everyone makes tradeoffs to make progress.
I'm willing to give up this, so I can get that. No one can have everything. Sales is about managing and packaging the tradeoffs the consumer is willing to make.
Disconnected: Most people don't know why they do what they do.
They will tell you the purchase was random, an impulse purchase, or they'll give you a simple answer. You need to dig and not accept their surface response. Most people live their lives at the moment and don't connect the dots. "I bought a mattress on an impulse." "So how long haven't you been able to sleep?" "Two years".
Lies: Everybody Lies.
You must be careful not to fall for the lies people tell themselves. If someone says, "I bought this product because it's important to be healthy." You should ask them to tell you three other things they've done recently to be healthy. They don't lie with malice or intent. They build a story that fits their world. We call this the cake layer of causality. Lies are a very complicated thing. This is about getting past the lying, past the cake layer.
You can read the full notes on the book here.
Interested in learning about how JTBD can be applied to product development? Check out:

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