Assessments - which can include interviews and practical assessments - should be tailored to the specific role you are applying for. The Person Specification which is attached to the job details, and which you need to address on your application form, will outline which criteria will be assessed at interview to provide guidance. If you are to do an additional task such as practical test or presentation as part of your assessment then you will be advised of this when you are invited to interview.
Pretty simple but it depends on the job you are applying for, do some research as you would with any job
Good, but all takes time, once you are offered the position can take upto 8-12 weeks due to the relevant checks to take place eg DBS and references.
Difficult, asked questions irrelevant to the job
It is worth calling the department you are interviewing for and asking as many questions pre interview as you can.
The process is very regimented and organised specifically to the job titles