I never much cared why Michael O'Hare left the show, save I always knew it wasn't because of his ability as an actor. I fully believe he played Sinclair the way JMS wanted, and that the so-called "woodeness" of his acting, was a choice, considering who Sinclair is and what he went through.
I second this as strongly as I can possibly do so.
O'Hare's portrayal of sinclair in the first season of B5 is not wooden in any sense of the word. Sinclair when we see him is a broken person. He's got what is possibly the worst case of PTSD in the history of the human race. And O'Hare's performance is a nuanced and sensetive portrayal of this.
I had no idea that Mr. O'Hare had passed-onand makes me a little sad. In many ways, his part in B5 was overshadowed by what followed in Seasons 2-5, but he helped lay the foundations of what became a great SF series and deserves to be recognized for that.