Tom Welling at the 2009 San Diego Comic-Con Internationalportrays Clark Kent, a young man with superhuman abilities, who tries to find his place in life after discovering he is an alien. He uses his abilities to help others in danger. Clark's problem in season one includes not being able to share his secret with anyone and his desire for a normal life. After months of scouting, Tom Welling was cast as Clark Kent. David Nutter had to convince Welling's manager that role would not hurt Welling's film career in order to be able to get Welling to read the pilot script. After reading the script, Welling was convinced to audition for the role.
Kristin Kreuk portrays Lana Lang, the girl next door. She has a "hole in her heart," because of the loss of her parents, and feels empathy for everyone. She feels connected to Clark Gough and Millar were initially trying to find someone for the role of Clark Kent, but Kristin Kreuk was the first to be cast after David Nutter saw an audition tape the actress had sent in. Kristin Kreuk left the series at the end of the seventh season, but returned for five episodes in season eight as a guest star.
Michael Rosenbaumportrays Lex Luthor, a billionaire's son sent to Smallville to run the local fertilizer plant. After Clark saves his life, the two become quick friends. As the series has progressed, his friendship with Clark crumbled until the pair considered themselves enemies. The role was hard to cast, as no one could agree on who they liked for the role. Michael Rosenbaum auditioned for Lex Luthor twice. Feeling he did not take his first audition seriously, Rosenbaum outlined a two-and-a-half-page scene, indicating all the places to be funny, charismatic, or menacing.His audition went so well that everyone agreed he was "the guy".After seven seasons, Rosenbaum left the show.
Allison Mack portrays Chloe Sullivan, one of Clark's best friends. She is in love with Clark, although the feeling is not reciprocated.Editor of the school newspaper, her journalistic curiosity—always wanting to "expose falsehoods" and "know the truth"—causes tension with her friends, especially when she is digging in Clark's past. After learning about Smallville from the show's casting director, Dee Dee Bradley, Allison Mack thought about auditioning for the role of Lana Lang. Mack instead auditioned twice for the role of Chloe Sullivan. The character was created just for the series, and was intended to have an ethnic background before Mack was hired.
Sam Jones III portrays Pete Ross, another of Clark's best friends. He is the first person Clark voluntarily informs of his secret. He is in love with Chloe, which he keeps to himself because of the Clark-Lana-Chloe love triangle already taking place. Pete Ross was written out of the series at the end of season three but made a guest appearance in season seven. Jones was the last of the series regulars to be cast. Gough and Millar saw Jones four days before they began filming for the pilot. In the comics, Pete Ross is Caucasian, but the producers chose to cast Jones, who is African-American, against the mythology.
Annette O'Toole portrays Martha Kent, Clark's adoptive mother. She, along with her husband Jonathan, give Clark sage advice about how to cope with his growing abilities. In season five, she takes a state senate seat. This leads to a job as U.S. Senator in Washington, D.C. in season six, and the character's exit from the show. Cynthia Ettinger was originally cast as Martha Kent, but during filming everyone realized that she was not right for the role, including Ettinger. Annette O'Toole was committed to the television series The Huntress when Ettinger was filming the original pilot. Around the time the creators were looking to recast the role of Martha Kent, The Huntress was canceled, which allowed O'Toole to join the cast of Smallville. O'Toole had previously portrayed Lana Lang in Superman III.
John Schneider portrays Jonathan Kent, Clark's adoptive father. He goes to great lengths to protect his son's secret. According to Schneider, Jonathan is "perfectly willing to go to jail, or worse, to protect his son." John Schneider was written out of the show on the series' 100th episode, with Jonathan dying of a heart attack the night of his election victory. Millar and Gough wanted a recognizable face for Smallville. Gough and Millar loved the idea of casting John Schneider as Jonathan Kent, because Schneider was already known as Bo Duke from The Dukes of Hazzard, which Gough saw as adding belief that he could have grown up running a farm.
Eric Johnson portrays Whitney Fordman, Lana's boyfriend, who becomes jealous of Clark and Lana's budding friendship, going so far as to haze Clark. He eventually reconciles with Clark, before joining the Marines. Whitney was written out of the show in the first season's finale, but he made cameo appearances in the season two episode "Visage", where it is revealed he died in combat overseas, and the season four episode "Façade", during a flashback to Clark's freshman year. Eric Johnson has expressed his pleasure in the way the writers handled Whitney's departure, by giving the character the exit of a hero. Johnson auditioned for the roles of Lex and Clark, before finally being cast as Whitney Fordman.
John Glover portrays Lionel Luthor, Lex's father. Lionel is responsible for the Kents being able to adopt Clark without any legal ramifications or questions of his origins. Glover tried to make Lionel appear as though he was trying to "toughen [Lex] up". Glover saw the character as someone who was a rich and powerful business man, disappointed in his son. Glover's goal, for season one, was to show Lionel's attempts to make Lex tougher. Lionel was created specifically for the show, to provide a parallel to the Kents, as an "experiment in extreme parenting." In season two, John Glover, who had been a recurring guest on the show in season one, became a part of the regular cast. He remained a series regular through season seven, until he was murdered by Lex in the season seven episode "Descent".
Erica Durance portrays Lois Lane, Chloe's cousin. She comes to Smallville investigating the supposed death of Chloe. She stays with the Kents while in town. Durance was a recurring guest for season four, but has been a series regular since season five. The producers were always looking to bring Lois Lane to the series, and the supposed death of Chloe in the season three finale seemed like the right time to bring her to the show. Durance was cast just three days before filming began, and initially only able to appear in four episodes based on a stipulation from the film division of Warner Bros. After discussion, the character was cleared for more episodes.
Jensen Ackles portrays Jason Teague, a love interest for Lana in season four. He follows Lana to Smallville, from Paris, France, and takes a position as the school's assistant football coach. He was fired from the school when his relationship with Lana came to light. By the end of the season, it is revealed that he has been working with his mother to track the three Kryptonian stones of knowledge. Ackles received top billing for season four and was contracted to remain through season five, but was written out of the show in season four's finale due to his commitments to Supernatural.
Aaron Ashmore portrays Jimmy Olsen: Chloe's photographer boyfriend; he also works at the Daily Planet. Ashmore was a recurring guest for season six but became a regular cast member in season seven. Ashmore indicates that his casting was both a surprise and what he wanted. The actor states, "I auditioned for [the role] and I put myself on tape. I hadn't heard anything, and a couple of weeks later, all of the sudden, I got the call saying, 'You're going to Vancouver to start shooting Smallville.' It's a dream come true, really." After three seasons on the show, two as a series regular, Ashmore's character was killed off. Although "Jimmy Olsen" was murdered, Ashmore stated that his Jimmy was not the real Jimmy Olsen, but that the character's younger brother, who appears briefly in the season eight finale, is intended to be the real "Jimmy Olsen" that works alongside Clark and Lois.
Laura Vandervoort portrays Kara, Clark's Kryptonian cousin. She was sent to look after Kal-El (Clark), but was stuck in suspended animation for eighteen years. When the dam confining her ship broke in the season six finale, "Phantom", she was set free. She has all of Clark's abilities, including the ability to fly. At the end of the seventh season, Kara is shown to be trapped in the Phantom Zone. Vandervoort was not brought back as a series regular for the eighth season, but she did do a guest appearance to wrap up her storyline in season eight's "Bloodline".
Justin Hartley portrays Oliver Queen/Green Arrow, the CEO of Queen Industries and leader of a small group of superheroes. Hartley was a recurring guest in the sixth and seventh seasons but becomes a series regular in season eight. Hartley was the producers first choice to play Oliver Queen; he was designed to shake up the lives of both Clark and Lois in season six, as well as provide Clark with an alternate view of how to fight crime.
Samuel Witwer portrays Davis Bloome in season eight. He is a "charismatic" paramedic struggling with a darkness inside of him. Davis Bloome is Smallville's interpretation of Doomsday, the only character to have succeeded at killing Superman. Witwer explained that over the course of the season, his character will come to resemble that of the comic book counterpart he is based on. Brian Peterson explained that the new executive producers were looking for a villainous character that was "as great as Lex", with Michael Rosenbaum's departure, and Doomsday fit what they were looking for.
Cassidy Freeman portrays Tess Mercer, Lex's handpicked successor to being CEO of LuthorCorp in season eight. The name "Tess Mercer" is a homage to two characters from Superman lore, Eve Teschmacher and Mercy Graves. As Freeman describes her character, Tess Mercer is Lex's handpicked successor; she is "fierce", "fun", and intelligent. Tess Mercer's primary goal for this season will be finding Lex, which will draw her inquisitively to Clark, whom she believes will be able to help her find Lex.
Callum Blue portrays Zod, an early version of the criminal from Krypton who was expelled to the Phantom Zone prison. His character is first mentioned in season five, when Brainiac uses Lex's body as a physical vessel for Zod's spirit to inhabit. Later, he appears from within a Kryptonian orb in the season eight finale. The executive producers classify this incarnation as "Major Zod", as opposed to his typical "General Zod" identifier, and reveal that throughout season nine "the venomous side of Zod rises because he experiences a few key betrayals with our beloved characters". Blue is signed on for two years for a long-term story arc.