Be generous with kindly words, especially about those who are absent. – Goethe
If you want to earn the trust and respect of those around you, be careful what you say about those who are absent.
When you find ways to (truthfully and accurately) praise those who are not there, you build trust and gain respect among those who are there.
Do the opposite however and you will poison the minds of those present.
Whether they consciously realize it not, they won’t know if they can trust you. They will wonder what you’re saying when they’re not around . . . .