I heard that before the series started Michael Weatherly was in Australia filming a movie where his character was Robert Wagner. He found out that that Donald Bellesario was in the area and crashed his dinner at a restaurant because he wanted to be on NCIS. Obviously he got the part and ironically Robert Wagner became his dad on the show.
Also, there have been a few new small characters on the last few NCIS episodes that may become a new team member.