In what is likely his final appearance on Fringe, Leonard Nimoy is back as William Bell. This is a photo from the second season finale.
Olivia is in the alternate universe in this scene from the second half of the season two finale. We learn a lot more about that world during the episode.
This is a scene from the second half of the two-part Fringe season finale. It depicts a conversation between William and Walter.

Fringe Season 2 Episode 22 Quotes

Walter: I should never have doubted you.
Bell: It's OK Walter. You always were as stubborn as a donkey with a nail in his head.

I'm trying to see this your way Walter, I can't. But, you did cross universes twice to save my life. So that's got to count for something right?