Hello Marco, very good questions indeed.
1. isn’t it expensive to get the whole team (I guess that would be the functional team that would take on the project…3 people roughly?) on a Design Sprint for an entire week?
Yes, it is expensive. However involving the project team from the start should help you avoid traditional waste during product development. More precisely: higher alignment between team members = less errors = less technical debt = more commitment and motivation = less overall stress and waste.
2. would you run this part of the process also for smaller projects or only for fairly big ones?
I would run Design Sprints for both big and small projects, evertime there are tough choices to make.
3. do you conduct the Design Sprints with your clients’ participation as well? or you first run an in-depth interview with your clients to collect precise“requirements” and only after start the Design Sprint?
We wouldn’t run design sprints without our clients. They need to be there because they are the most important part of the decision-making process. If they can’t join, ask them to be there for the key moments: day#1’s “ask the expert” (focus on the business and project goals) and day#3’s “decide” (focus on what to prototype)… last but not least day#5’s interviews :)
4. if you have a successful response from the Users interviews during the last day of your Design Sprint, do you still normally invest more time prototyping the product the week after before starting the actual development?
Normally, yes. More precisely we tend to do that in the first iteration of the product development while developers focus on technical analysis.
I hope I have answered to your questions — let me know if you have further things you’d like to know or if you want to share how you guys work inside your company / organization.