Characterization
Despite his intelligence, Zack is unsure of himself and though he has come up with crucial insights vital to some of the team's cases, he is unable to forcefully express his opinion to Dr. Brennan. This may be due to romantic feelings toward her. When he discovered Temperance's own Forensic Anthropology professor had become her lover, Zack repeatedly wondered aloud whether he might enter into a similar relationship with Dr. Brennan. He was quickly disabused of the notion by his colleagues.
Zack appears to have an on and off-again relationship with "Naomi, in Paleontology", despite hints in the first season she was dissatisfied with his sexual prowess. In the third season, they accompany each other to the annual Jeffersonian Institute Halloween party, agreeing to dress as a cow (Naomi in front, Zack in back) ("The Mummy in the Maze") and in "The Parts in the Sum of the Whole" he talks about her again. Zack has no problem making inappropriate comments about others' personal lives, and has asked Agent Booth for advice on sex and women, requests which Booth characteristically ignored; and at one point threatened to "take out gun and shoot between the eyes" if he continued to ask him those questions. Angela answered his question once, (on one occasion even recommending him to "reap the benefits of sexual wisdom"), and Hodgins gave him a pocket-sized Kama Sutra to help him out, revealed in the season three finale ("The Pain in the Heart").
Very little is known about his childhood. In "The Wannabe in the Weeds" episode from the third season, it was discovered Zack was in singing classes during his childhood as a way for his parents to help integrate him socially. Though it did not appear to work, he demonstrates his talent when Hodgins doubted him. He has fond memories of receiving his first microscope ("The Girl with the Curl"), and when he was six, he had a pirate eyepatch, ("The Man with the Bone"). He said he used to play horses as a child and has a Michael Jackson glove. It is also known he attended a private high school ("A Boy in the Tree"). His interests include model airplanes ("The Killer in the Concrete"), watching basketball ("The Soldier on the Grave"), and science fiction: Firefly ("The Man in the Fallout Shelter"), Lord of the Rings ("The Woman at the Airport"), Star Wars, Star Trek, Stargate, Battlestar Galactica ("The Superhero in the Alley"). He does not dance, because he's been told he "looks like a marionette in a windstorm" (The Man in the Wall). Although well-meaning, helpful, and friendly, when a situation calls for social interaction or intuition, he is often lost. Further evidence of his social ineptitude can be seen in the frequent, on-screen coaching in social matters he gets from Jack and Angela. He is very literal-minded, and is often confused by colloquial expressions or metaphor, despite his high intelligence. His attempts to use such expressions meet with mixed success, such as referring to a skull he'd cleaned as being "clean enough to eat off of" ("Two Bodies in the Lab").
In the episode "The Killer in the Concrete", Booth is on the phone with Zack and Dr. Brennan while looking for "Icepick" at a model airplane enthusiasts gathering. Unaware of Zack's interest in planes, Booth comments every "airplane freak" in the area was at the event, and Zack corrects him by saying the enthusiasts prefer to be called "pilots." Zack mentions forensic anthropology is only one of his areas of study, another is applied engineering, and he is extremely adept in practical aeronautics.
As seen in the episode "The Man in the Fallout Shelter" Zack has a large family and greatly values them, remarking the true meaning of Christmas is "Brothers, sisters, nieces, nephews; 40 people who love you and are happy to see you." Also in the first season, he reveals to Dr. Brennan he uses his vacation time to visit his family. While he does not necessarily enjoy these visits, he goes because they love him ("The Man in the Wall").
Even in the asylum, Zack apparently still has some degree of contact with his old colleagues, Hodgins occasionally visiting him with puzzles for him to solve and Sweets having formed a diagnosis of Zack, (although he is unable to reveal that Zack is not actually 'crazy' due to doctor/patient confidentiality); Booth's dream sequence in "The End in the Beginning" implies that Booth is aware on some level that Zack is innocent, but this has never been explored in the real world.
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