This guide will let you know how to talk with other characters in GTA 5.
You will need to play GTA 5 online to interact with other characters. If you are playing as Franklin, then you can talk with Trevor and Micheal.
Go closer to Trevor; they can have a chat with one another.
You can also interact with the son of the big mafia. Go to him and ask where his father is. He will reply that his father is busy importing and drug dealing.
Follow the same method to chat with Jimmy and other personal guys. They will start following wherever you go.