M.P.
My grandson showed an interest around 2 1/2 but he was slow about learning. I don't think anyone did anything special to start his interest. He does have an older sister to copy. He started out pointing to his diaper when he was wet and we would take him into the bathroom to change him. We talked about how one day he'd be a big boy and could do it in the toilet just like Sissie. He was finally trained a month or so after his 3rd birthday. It seems he finally caught on when his mother stopped using pull-ups, even the ones designed to feel wet, and either left him naked or in big boy pants. One day he arrived at my house in big boy pants and had an accident. I only had pull ups to put on him. Another time I didn't have anything to put on him and he stayed in his wet clothes for a couple of hours. A few days later the same thing happened but the accident was during speech therapy. I think that he seldom had accidents after that. We considered him trained. And we praised him every time he had any sort of success. Even when he came to tell us that he was wet. "That's good to notice when you're wet! I bet you can get to the bathroom quicker next time." sort of thing. Eventually he did.
I think their mother waited until his sister indicated an interest before she did anything too. She did read her children's books about using the potty even before she showed an interest.