Mine consisted of, first, filling in a detailed 'developmental history' questionnaire which spanned my life experiences from birth - even before birth, actually, as there was a question about my mum's pregnancy. It took me about three weeks to complete it (doing bits at a time) as I wanted to make sure it was as accurate as possible, and included a good range of examples that were relevant.
This was followed by a face-to-face interview with two psychiatrists who asked me questions using the questionnaire as a starting point, mainly around how my life was impacted by the traits and so on that I had described.
I was relieved that there was no role-playing or anything like that! It was just talking.
At the end of the interview I was given a diagnosis of Autism, and then they took time to point me to resources and support; they also checked whether I wanted the diagnosis to be shared with my GP.
There was a waiting list even though I was going private, of around three months.- the service I used also does NHS diagnoses.
I'd suggest taking time to 'shop around' and look at how accessible websites are; and how helpful they are if you phone them, to make sure you are choosing a service that is right for you personally.
Wishing you the best in your route to diagnosis, @MoreLettuce