Our words have power.
If we speak ill of someone, we create incentive to believe in that narrative, so that when we are around them we feel the need to treat them the way we’ve spoken of them behind their back.
If we treat them differently and smile to their face, our inner self knows and convicts us of the incongruity between our present actions and what we’ve said of them before.
Avoid the self conviction, and only speak well of others.