Each of them has special characteristics, but if he still working and looking for a problem solving so he feels he is still live whether he researcher or expert.
When my supervisor called me scholar, I was so surprised; am I able to be called as scholar; scholars are some giants, that I am not, Researcher is a good term as it makes me humble, as someone, who is trying to re search, and learn, and find answers; expert is some big thing, in big scheme of life; that I am not.
The use of such terms depends on the context. If a person is attempting to explore the truth by applying various methods and techniques of research methodology, the former may be addressed as a Researcher in that context. However, if he/she is attempting to solve a problem by applying the existing and proven knowledge, then the person may be addressed as an Expert.