Fitting in as the others have mentioned is number 1, shortly followed by how you would interact with customers/clients, then flying ability. In that order. All three of those areas have to meet a minimum standard.
In terms of the interview, some topics can include from my knowledge/experience:
- Aircraft speeds/limitations etc
- AIP knowledge such as VMC, ENR 1.1, flight planning, weather, alternates
- Fuel planning requirements CASR/MOS P135 etc
- Typical CPL flight test ground component items
- 'What would you do' type questions of typical issues faced in day-to-day operations
- Last light/latest departure time calculations
- Passenger operations- including pax brief requirements, seating capacity, emergency equipment, and items along those lines.
The most important thing in my opinion is to demonstrate sound legal/technical knowledge in the interview, along with flying ability on a check flight, whilst clearly projecting your ability to fit into a small, team-based environment that won't ruffle the feathers of the existing pilot group, whilst being capable of turning it on for customers/clients.
Hope this helps.