I was contacted 4 days after the application deadline for an interview, which had a 72 hour deadline. First round entailed a one way video interview (speak to a camera, responses are recorded and viewed afterwards by HR). All behavioural questions, unless you count 'knowledge of the SDLC' technical - in that case, there are 8 behavioural and 1 technical questions. The behavioural questions are wide ranging - from a time you failed to complete a task and the lessons learned, to how you've used the SDLC in your projects. There's no one real way to prepare for this - you can google various questions and practice them, but at the end of the day you just need to be able to talk confidently. Follow the STAR(LA) method - you're typically given 30s - 1min to prepare, and up to 4 minutes to speak. Unfortunately, I did not make it past round 1. But I know that round 1 is often touted as the hardest part in this process - the next steps include a dinner social with the partners, and two in person interviews the following days with senior managers/partners (mainly for fit and culture). Note that Deloitte assigns a "Relationship Manager" to you in the process - typically an employee in the role, who can tell you about day to day life and the various company perks. This helped, as the recruiter setting up the interviews was clearly from HR and had many candidates to deal with.

I went through two interviews. The first interview included the behavioral part and case part. The second interview was with a managing director, and the entire interview was about solving a M&A case.