First thing you should do is read Luke's post above you will learn everything you need to prepare for the actual interview. As for working for them, well....
Highland consist of 3 Jetstream 31's and 3 Cessna 406 and the flight crew work in either the commercial operation or the fisheries operation.
Two of the 406 are owned by the Scottish Fisheries Protection agency and used exclusively on Fisheries Patrol. They consist three crew (captian, fo and a radar operator) flying around at low level for up to 5 hours chasing naughty fishing boats. The job requires a good amount of VFR stick skills as a 45 degree steep turn at 200 ft whilst dodging kamakaze seagulls is not for the faint hearted!. There are no early starts, nearly every weekend off with the roster usually 2 weeks on 1 week standby. The crew in the fisheries are an easy going bunch, and you will be expected to spend 5 days away from base in such delightful places as Sumburgh and Stornoway, so if you have a young family make sure they are of the understanding kind. As this is a government contract, they stipulated that the flight crew are ring-fenced within Highland and conduct no other flying. So if you are employed as a fisheries pilot, that's all you'll do in Highland.
The commerical flight crew are either rated on the Jetstream or the 406 (or sometime both). The main contract operated out of Inverness is delivering newspapers to the Western and Northern Isles. The 406 is used to cart the papers to Northern Isles and the Jetstream takes the papers to the Western Isles. Once the Jetstream gets to Stornoway and dumps it's load (and you will be expected to help unload) it then operates a schedule between Stornoway and Benbecula twice a day before returning to Inverness. As there is only one crew involved, then this leads to some long duty days. The 406 drivers have an slightly easier life as once they finish their paper round (about 10-11am) they pretty much have the rest of the day to themselves. However, the 406 is used as a Single Pilot aircraft (unlike the fisheries) and like all single pilot ops, great fun on nice days, bloody hard work on the not so nice days. The other main contract is operated out of Warton, exclusively on the J31. This is a monday-friday run down to cambridge and back and again can include some fairly lengthy duty days. This contract is crewed locally, but sometimes inverness crew have to go down for a week to cover holidays/duty/sickness. The last Jetstream is used for Charter work, and as is the nature of Charter work, can either be very busy or gathering dust in a hangar. Most charters are based around the oil industry so involve trips to Norway, Ireland and Norwich! Commercial operations requires early starts, working at the weekends and a flexible attitude to rostering, no different to any TP operator.
Whatever operation you are being interviewed for, the type rating is paid for by the company and you are bonded for 3 years, with the bond decreasing a set amount each month. If you have all the ticks in the boxes, then time to command can be relatively short in the commercial sector. However, fisheries tends to be dead man's shoes if you're looking for you're bump to the left hand seat. Pay is about average for a TP operator, a good pension scheme, no sector pay and you pay for your own medical.