Interview is a pleasant experience. They want to get to know you, see if you fit in with the company culture, which IMO is professional but relaxed, typically Scandinavian. Prior to interview they send you links to online personality test and Matrigma (visual, logical reasoning) test. You get the results at the interview. Once again, they want to see you are neither an axe-murdering recluse, nor an arrogant sky god
If you're DEC, expect a sim check too. I heard a rumour they may start this for FO also, but the sims are working hard already, so this may be just a rumour. Apparently, failure rate for DEC in the sim check is 50%, which makes me look good
but in all honesty it is pretty straight forward, you get all the info in advance to prepare, and those who fail usually do so based on CRM issues rather than their flying ability.
Fastidious Bob... All contract Captains earn the same, on average EUR 10,000 per month gross. Your agency will deduct social taxes for the country in which you are based (this is a whole other can of worms!) but if you're UK based, it's circa £500 per month, the rest is paid to you gross, EUR 9,500 and you sort your own taxes out with Mr HMRC on a self assessment basis. I would work on a monthly net of £6,000-£6,500. Basic salary is EUR 7,500 allowances/expenses of EUR 1,300 and flight pay of EUR 17 per hour, so a lean flying month has little impact on your take home
Hope this helps. I can only but recommend NAS, but of course that is based on my own experiences.
Good luck and fly safe!