Just wrapped up my Slack loop after coming back from a two-year career gap, and the behavioral rounds were honestly the part I was most nervous about. Sharing specifics in case it helps someone.
Slack runs two dedicated behavioral rounds in the onsite. Each is 45 minutes, and they're structured around what they internally call their values (focus on authenticity, default to transparency, etc.). The questions map pretty closely to those, even if the interviewer doesn't name them.
Actual questions I was asked or heard about from others in my cohort: "Tell me about a time you had to share bad news with a stakeholder. How did you decide what to communicate and when?" "Describe a situation where you disagreed with a technical decision your team made. What did you do?" "Tell me about a project that didn't go as planned. What would you do differently?" "Give me an example of when you had to work across teams with very different working styles." "What's something you changed your mind about in the last year, professionally?"
That last one caught me off guard. It's not a standard STAR question. They want to see intellectual flexibility, not just execution stories.
What they're evaluating under the surface: how clearly you communicate, whether you own your failures, and whether you respect others' perspectives. People who give perfectly polished "I did everything right" answers apparently land flat. Admitting you got something wrong and showing what you learned is actually the stronger signal.
For prep: I used a structured story bank method where I pre-wrote out 8-10 STAR stories with different themes (conflict, failure, influence without authority, big scope, fast-paced decision). You won't need all of them but having them ready lets you adapt quickly.
Slack's behavioral bar is real. This isn't a formality round.