I interviewed at Square last quarter (senior engineering role) and also coached a few people who went through the loop this year. The behavioral round at Square is more structured than at most companies I've seen -- they're very clearly mapping to their operating principles.
Square talks about being "customer obsessed" and "moving with urgency." Those aren't just buzzwords in the interview. The behavioral questions are basically: show me a time you acted on one of these principles under pressure.
Questions I actually got asked or heard reported: "Tell me about a time you had to make a decision with incomplete data and something significant was at stake." "Walk me through a project where you disagreed with the direction but had to execute anyway. How did you handle it?" "Describe a time you failed to ship something on time. What happened and what would you do differently?" "Tell me about a situation where you had to push back on a product requirement for technical or ethical reasons." "When have you had to communicate a complex technical tradeoff to a non-technical stakeholder?"
A few patterns I noticed:
They lean into failure questions. Not "give me a challenge you overcame" but actual failure. Be specific and own the outcome. Candidates who pivot failure questions into humble-brag success stories get lower marks.
Cross-team collaboration comes up heavily, especially since Square (now Block) has multiple product surfaces. They want to know you can work across orgs.
Speed vs. quality tradeoffs are a recurring theme. Payments can't have bugs, but shipping matters. They want to see you hold that tension consciously, not pretend it doesn't exist.
Star method works fine structurally. Just make sure your T (Task) is specific and your A (Action) has enough detail to be credible. A lot of candidates rush through S and T to get to what they did. The interviewer cares about the context -- it signals you actually understand the situation you were in.