Processing of personal data must always be fair as well as lawful. If any aspect of your processing is unfair you will be in breach of this principle – even if you can show that you have a lawful basis for the processing.
In general, fairness means that you should only handle personal data in ways that people would reasonably expect and not use it in ways that have unjustified adverse effects on them. You need to stop and think not just about how you can use personal data, but also about whether you should.
Assessing whether you are processing information fairly depends partly on how you obtain it. In particular, if anyone is deceived or misled when the personal data is obtained, then this is unlikely to be fair.
In order to assess whether or not you are processing personal data fairly, you must consider more generally how it affects the interests of the people concerned – as a group and individually. If you have obtained and used the information fairly in relation to most of the people it relates to but unfairly in relation to one individual, there will still be a breach of this principle.
Personal data may sometimes be used in a way that negatively affects an individual without this necessarily being unfair. What matters is whether or not such detriment is justified.
You should also ensure that you treat individuals fairly when they seek to exercise their rights over their data. This ties in with your obligation to facilitate the exercise of individuals’ rights.