After last week’s explosive episode in which Peter learns the truth about himself and turns his back on Walter, “Fringe’s” producers knew they had to explore what effect this would have on the already fragile Walter.

The show’s Executive Producer Jeff Pinkner explains, “The last episode that aired, Peter learned he wasn’t from our universe … and turned his back on Walter [John Noble]. So we knew we wanted to tell an episode that really explored — we have this phenomenal actor in John Noble and this great character in Walter — how that affects Walter before we plunge forward into the end of the season. We came up with a narrative device to really explore Walter’s feelings.”

In the episode, airing tonight, Walter tells a story that casts Olivia (Anna Torv) as a hard-boiled detective and Peter as the tough guy she’s searching for. “We felt it was a way to get across Walter’s [frame of mind] and where his head is right now with his son missing,” fellow EP J.H. Wyman says. “There’s a very good reason why it’s noir. It’s because of [Walter's] own history and things like that, which you’ll see.”

The musical number has more to do with sweeps week and less to do with Walter’s emotional state though. All this week, “FOX Rocks,” will feature some musical element on the network’s most popular shows. (Thursday’s “Bones” murder, for instance, involves a rock ‘n’ roll fantasy camp).

As far as the music, Pinker said, “It really deepened and blew the episode out even further in the direction we were already taking,” he says. “… This episode provided an opportunity to hold a mirror up to Walter’s perspective of the world and these individuals — Olivia, Peter and Astrid [Jasika Nicole] — that he interacts with. We get his fractured take on the world and his condition now that Peter has left. The music really supports the storytelling.”

Here’s a sneak peek at “Fringe’s” film-noirish episode, “Brown Betty,” which airs at tonight at 9 p.m.