When I talk to a dream character in a lucid dream, I talk to them just how I would in real life. Like Puffin said I too have not tried using telepathy but that seems like it would be a more interesting way to get dream characters to do what you want them to do. I do actually notice a difference in the way I talk to dream characters, in a non lucid I seem to talk to them like a deer in the headlights. I just seem to agree with them a lot, and just talk a lot more naive and stupid I guess. I don't find it hard to talk to them, but if you seem to be having trouble you just need to relax and just talk to them how you would talk to anyone in real life. Just be casual about it! DC's don't bite!

Hope that answers your question, good luck!