In the episode "Preggers", Mark Sinacori performed the role of the teacher sitting alone at the break room table as Will Schuester approached to sit down as there was a seat available as not to intrude on Emma and Ken who were having lunch across from this teacher.
Will looked up and saw this teacher, who smiled at him and Will decided not to sit with him as he seemed "creepy" to sit with, moving on to ask Emma and Ken if he could sit with them.
The back story to this character is that it wasn't just a picture picked extra or was one of the extras who got to be teachers that day that got to audition to get that featured spot. The writer of the episode, Brad Falchuk, was a member of the exclusive Red Sox fan group Sons Of Sam Horn (SOSH) that had ties to Mark's high school Central Catholic High School in Lawrence, Mass (as many of its members were part of it) and had seen Mark Sinacori online through his postings on you tube and other video sites since 2006. Mark was indeed an actor, who others in that group who went to his high school had thrown down saying he couldn't write, create, sing, or act in any way. Brad personally called for Mark to be in the episode and Mark agreed to it as he loved playing character parts, no matter what they were.
Mark was told he would be a creepy teacher on the show. The day of filming, on Mach 20, 2009 at Paramount, Brad Falchuk told Mark to play the character like a creepy math teacher with a secret.
9 episodes later, the episode "Sectionals" opened with the students talking about Mr. Sinacori the creepy math teacher who always sang down the halls. Although he was never called back to the show, Mark's character in the breakroom now had a name. Not many Glee fans know of this trivia but that character he played in the breakroom was indeed Mr. Sinacori the Creepy Math Teacher that the kids were talking about weeks later.
Even though there has been some fanfic where Mr. Sinacori was mentioned or portrayed in over the years by fans of the series Glee, the show never brought Mark back in the role or had him back to have him even referenced as that character in a scene. Mark would have been up to playing the role again anytime he was needed throughout the duration the show had filmed.
