If you must talk about someone, try to imagine them overhearing your conversation.
Recently, someone tore apart a comedy sketch I wrote. This person repeatedly made unkind comments about my work, without realising that I was in the room.
Of course, I didn’t take the comments to heart. A large part of being a comedian is withstanding negative and rude feedback.
When someone asks for your opinion about a person, or the quality of their work, speak as if they are in the room. Assume that they will eventually hear what you have said. It’s amazing how much people talk.
Whether your opinion is positive or negative, it is always possible to be kind.