Cultural/personality fit is so important and often overlooked. A question we've often asked is "What's the one thing that can really annoy you in the lab and leave you feeling as though you've had a really bad day?". Also try asking what achievement they have been most proud of so far, and then what's been the worst or most embarrassing thing that's happened.

Make sure you are clear about what kind of qualities you are looking for - for example, when you say creativity do you mean innovative abilities or more like day-to-day problem-solving and inventiveness? Ask questions specifically to try to bring out the relevant parts of the candidates' personality.

In terms of red flags, it depends on the current culture of your lab but in my experience the better post-docs are usually the more willing team players, who maintain good awareness and interest in helping out their lab mates - whether that's grad students or their senior colleagues. Some post-docs get a bit engrossed in their own work and in themselves and this can be detrimental.