Blurting out [brutal honesty] is too hard to distinguish from passive agressive smallness; [you’d] best have done some work first to ensure you don’t have some passive agression leaking out somewhere in the first place.
The right way to do this is to not say too much at first, but to also actively seek out those who value the truth. Sharing the truth [is] like sharing intimacy — it’s best done consensually.
Look for those who will take substantive stands, and filter away those who are just saying things to curry favor or fit in. Welcome even those you might disagree with. Admit when the other has a point, and see what they do with that. People reveal whether they really care about the truth rather quickly. Those who do not care often reveal that unknowingly.
The only tact necessary is to withhold an opinion by claiming ignorance or unfamiliarity with a topic and to sincerely respect others. One can usually do both truthfully — very few people are the world’s best experts at a given thing, and few can claim to know all the details of another’s situation. If you have the humility to understand this as the truth, then you can usually exercise tact without resorting to untruthfulness.