As a result of Lee's inclusion, "Neither Here nor There" is the first episode to introduce his portrayer Seth Gabel, formerly a recurring actor, as a main cast member.

When forensics comes on the scene, Olivia and Astrid (Jasika Nicole) appear and take the agent's body without revealing their intentions.

Olivia agrees to bring Lincoln onto the case, and shows him that there have been roughly thirty victims in the last several days, who have had no obvious connection or signs of death, other than that their hearts stopped.

During the episode, Walter is shown to be agoraphobic and unwilling to leave his lab after Olivia arranged his release from a mental asylum.

He becomes panicked when he briefly sees images of a man, Peter, in the lab; Olivia attributes this to Walter lacking any foothold in reality.

That evening, Walter is preparing for bed while September, outside, readies a device he has assembled to wipe out the remains of Peter's existence.

Former guest actor Seth Gabel, who played Lee, joined the cast as a series regular for the fourth season.

[6] When discussing the season's arc, Fox and Warner Brothers wished to have Peter return as soon as possible, as they knew he was popular with fans.

Pinkner and Wyman responded that to "really establish his absence [it] will take a little while", but knew that a longer wait would lead to audience frustration.

[7][8] The episode's opening credits featured a new amber-tinted sequence, which listed new fringe science terms like "Psychometry", "Gravitons", and "Existence".

[12] The episode also featured a one-time guest appearance by actor Joe Flanigan, who played Lee's partner Robert Danzig.

[14] The scene in the diner where the Observers discuss Peter's existence featured "California Dreamin'" by vocal group The Mamas & the Papas.

[7][9] Other songs featured in the episode included "Rockin' Robin" by Bobby Day and "Cinderella in the Palace" by Sergei Prokofiev, the latter performed by the National Symphony Orchestra of Ukraine.

"[9] He added that after the opening exchange between Olivia and Fauxlivia, the episode "settles into a fairly typical" Fringe formula involving a "villain of the week" and a freakish event.

[9] Andrew Hanson of the Los Angeles Times praised Anna Torv's two performances and believed "the containment built around 'the Machine'" may "have been my favorite part of the episode.

"[19] Richard Edwards of SFX gave the episode 4 out of 5 stars, writing that it "chooses to mostly ignore exploring the implications [of Peter's disappearance] on the overall story arc in favour of a fairly run-of-the-mill (by Fringe standards, at least) case of the week.

Even the potentially scintillating scenes between the alternate incarnations of the characters fail to materialise, aside from a couple of stilted encounters between the two Olivias.