He proceeded to guest star on popular shows such as
Judging Amy and
Close to Home. Gallner joined the cast of
Veronica Mars as Cassidy Casablancas toward the end of the show's first season, in May 2005, before becoming a season regular during the show's second season. After playing
Bart Allen in the
Smallville episode "Run", Gallner reprised the role on the January 18, 2007, episode "Justice" and the season eight finale. He also guest starred on
Cold Case as a teenaged gunman in the episode "Rampage", and appeared in the independent film
Sublime. He also guest starred in an episode of
Bones as Jeremy Farrell, the suspected murderer of a child beauty-pageant contestant. In 2007, he guest starred in an episode of
Law & Order: Special Victims Unit titled "Impulsive" as a high school student who accuses his teacher of
statutory rape, as well as the only survivor of a family slaughter—which initially rendered him the prime suspect—on "
Homewrecker", the season 3 premiere episode of
The Closer. He also played the lead role in the horror film
The Haunting in Connecticut, playing a cancer patient who is tormented by evil spirits. He appeared in the
independent film Cherry, filmed on the campuses of
Kalamazoo College and
Western Michigan University, along with several locations in downtown
Kalamazoo, Michigan. He plays Aaron, an Ivy-League freshman who falls for an older woman while becoming the object of affection for the woman's young daughter. Gallner played Quentin in the
2010 reboot of A Nightmare on Elm Street; his character is the love interest of main character Nancy. Gallner played the brother of
Aaron Paul's character in the 2012 film
Smashed. Gallner played Zach in one episode in the fourth season of
AMC's
The Walking Dead. It was initially reported that he would be a recurring character, but his character was killed off instead. In 2013, Gallner co-founded the independent production company Minutehand Pictures along with Oliver Thompson and Bay Dariz. The trio produced the feature film
Welcome to Happiness, in which Gallner portrays Woody Ward, a children's book author with a secret door in his closet. In 2014, Gallner played the role of Kurt, the antagonist to lead character Sam, in the
independent film Dear White People. That same year, he acted in
Clint Eastwood's war film
American Sniper. Gallner had roles in several independent films throughout the 2010s and 2020s. He starred in the critically acclaimed comedy
Dinner in America, which premiered at Sundance in 2020 and earned him praise for his performance. The film struggled to find an initial audience but developed a
cult following in 2024 after it gained popularity on TikTok. In 2022, he starred as
Vince Schneider in the
fifth Scream film, directed by
Matt Bettinelli-Olpin and
Tyler Gillett. He also had a significant role in horror film
Smile, for which he reprised his part for a
sequel in 2024. In 2023, he led the thrillers
Mother, May I?,
The Passenger, and
Strange Darling. For his participation in the horror genre, he has been labeled a
scream king. In the upcoming film
Carolina Caroline, he will be reunited with
Dinner in America director
Adam Carter Rehmeier. ==Personal life==