My first interview felt reasonably informal.
At the time I was shopping for an agency and was interviewing them as much as they are interviewing me.
It was 3 years ago - precovid and they had a quick look round my home.
We talked for about 2 hours, about all aspects of my suitability as an adopter and the types of children the agency dealt with.
The SW was 10 mins early (a common thing I subsequently have learnt) and I'm a late person who can get ready in the last 10 mins so I remember feeling my hair was a mess and I had no makeup on and I was on the back foot!
It went well I felt, but I wasn't ready yet so waited 3 years until I began the process properly (with the same agency).
I'm now in stage 2 and things I said 3 years ago get parroted back to me (it's a completely different SW).
It's a bit frustrating as I've researched a lot in those 3 years and I'm not as naive as I was about the challenges of adoption.
So don't get lulled into thinking it's an informal chat - the SW obviously makes extensive notes which stay on your file.
I've a terrible memory and a tendency to over-share and be glib, so I feel at a disadvantage that the meeting from 3 years ago is so well-documented and I don't have a clue what I've said! 
I've made lots of life changes in that time in preparation to adopt, and obviously the pandemic changing a lot of things, so it's annoying to not be given a clean sheet to start with. I often have to correct the information they have about me because so much has changed.
So all in all, not as informal a chat as it first appeared to be!!