I'll be honest, I wasn't expecting the behavioral portion at Costco to be as substantive as it was. I went in thinking it would be quick and surface-level. It wasn't.
I did an onsite for a senior software engineer role. One full round was dedicated to behavioral, run by someone who turned out to be a team lead (not the direct hiring manager). The questions were pretty clearly mapped to Costco's actual cultural priorities, which you can kind of reverse-engineer from what they ask.
Questions I got (paraphrasing): Tell me about a time you had to make a technical decision with incomplete information and a tight deadline. Describe a situation where you disagreed with a stakeholder's direction. What did you do? How do you balance moving fast versus making something maintainable? Tell me about a time you had to simplify a complex technical problem for a non-technical audience. What does good member service mean to you in the context of software engineering? (this one surprised me)
That last question is the Costco-specific one. They're a member-first company and they want to know you understand what that means operationally, not just philosophically. I talked about reliability and trust, about systems that don't go down during peak member moments. That seemed to land.
The format was STAR-style. They prompted for Situation/Task/Action/Result without using those words. When my answer was vague they asked follow-ups like "what was the actual outcome" or "what would you do differently." It felt genuinely curious, not adversarial.
Tip: don't underestimate this round. At Costco I got the sense that a strong behavioral round can carry you over a so-so coding round, more than I'd expect at a pure-tech company. The team culture stuff is taken seriously at the management level.
Prepare 5-6 strong STAR stories. Know your trade-offs and your regrets.