The trend in all modern Trek series as to find a group of actors to play the characters, sign them to seven year contracts, and never prepare or expect them to leave the series (unless they need to shake things up radically). Why? Why not hire the actors year by year and replace characters virtually every season? Like Law & Order. Characters and actors come and go but the show continues on. It would be alot more realistic. In the modern day military who has ever heard of the same captain staying in command of a ship for 7 years? Or four years for that matter? You get an ever changing crew dynamic with lots of opportunities for different interactions.