I believe Michael O'Hare was Directed like that, and although it does come off as too stiff and unimpressive early on, (because you don't know the Character), I think he does a perfect job of portraying "A Man with a hole in his mind, with Survivor's Guilt, and a tiny bit of a deathwish/Challenging Death Fetish". The portrayal works perfectly for me on rewatch, after knowing the character, and examining the nuances of the performance.
Bruce Boxleitner? That really surprises me. Although, alot is owed to the Writing, the actor had to carry the weight of the Arc. I never would've thought it ahead of time, or when he first walked onscreen in B5, but, I think Boxleitner, too does a fantastic job. John comes in as a fresh faced jolly guy, and as the weight of the Arc weighs on Sheridan, he changes, becomes far more serious and more mature than 4 years would age you in everyday life. John Sheridan is a complete different person by the time of the Penultimate episode (Sleeping In Light, of course is after a 20 year gap)
Claudia Christian, yea, she's not Meryl Streep, but, I enjoy Ivanova's portrayal, and I think she knocked several of her Dramatic scenes out of the park (Hour of The Wolf, I shoulda Boffed him at least once, Sitting Shiva, Talia's betrayal. Many people think "Hand of God" speech is a failure for her, but, I'm OK with it, personally).
Pat Tallman, again, not a great actress, but, she portrayed Lyta perfectly for me, and JMS decided after he saw her In NotLD Remake that she had telepath Eyes, and created the character specifically for her. (Though, there were a few embarrassing moments for her, IMHO)
I personally think Mira Furlan is an awesome Dramatic Actress, and she has so many awesome performances and Deliveries throughout the Series. I'm always surprised more people don't mention her along with Peter Jurasik and Andreas Katsulas