As fsloth says below, JMS knew every plot point from day 1 and had trap doors for every character. If an actor's contract fell through after 2 seasons, they did

As fsloth says below, JMS knew every plot point from day 1 and had trap doors for every character. If an actor's contract fell through after 2 seasons, they didn't have some emergency, they enacted the back up plan (Talia being replaced with Lyta). The shows lead had mental health issues and was also replaced after season 1 with some later cameos. It seemed like it was obviously meant that way. Even the super fans didn't know until recently when the actor died. Yes B5 had some painful filler episodes and the acting was both amazing (G'Kar, Londo, Delenn) and at times bad, but overall the show is the one show that has stayed with me since Kindergarten. Just like TNG episodes (the drumhead and measure of a man) served as mini courses in morals, ethics, and philosophy for kids, Babylon 5 was the same, but more memorable to me as the arc was 5 seasons, two spinoffs, and like 5 TV movies. I learned all about politics, facism, standing up for what's right, leadership, really cool spaceships, the impact of discrimination, and diplomacy (kind of :)). A great show to watch at an impressionable age.