2000–2007: Film debut and early roles Simpson made his film debut at age 25 when he played Noah in
Loser (2000), a teen romantic comedy directed by
Amy Heckerling. This was followed by a supporting role in the
Stephen King miniseries
Rose Red in 2002, and appearances on television shows such as
24,
NYPD Blue,
Cold Case,
Carnivàle, and ''
It's Always Sunny in Philadelphia, where he played the recurring character Liam McPoyle over several seasons, beginning in 2005. Film credits during this period included the sports comedy Herbie: Fully Loaded (2005), the revisionist Western Seraphim Falls (2006), and the critically acclaimed David Fincher thriller Zodiac'' (2007). In an appraisal of the latter's final sequence, where Simpson appears as Mike Mageau, a surviving victim of the real-life
Zodiac Killer, Jim Emerson of
RogerEbert.com wrote, "Mageau … is nearly a ghost, a deeply wounded soul who is 80 percent certain of his own certainty but, like everyone else, wishes he could be sure".
2008–2015: Stage, television, and film work In 2008, Simpson starred as
Philo Farnsworth in a production of
Aaron Sorkin's
The Farnsworth Invention on
Broadway. His portrayal of Farnsworth was described as "superb" by the
Chicago Tribune, and earned him a
Theatre World Award. That same year, he made the first of several appearances as Lyle, a fictitious intern, on
The Late Show with David Letterman; a role he frequented until November 2009. During that time he made guest appearances on
CSI: Crime Scene Investigation,
My Name is Earl,
House, M.D., and
Psych, and played supporting parts in the 2009 comedy
The Invention of Lying—the directorial debut of
Ricky Gervais—and the big-budget romantic comedy
Date Night (2010). Next, he appeared in one of the principal roles—
Dr. Lloyd Lowery—on the
A&E crime drama series
Breakout Kings, which ran from 2011 to 2012. The show drew a mixed reception, but critics agreed that Simpson's performance was its best asset. Simpson's next projects were the films
Abraham Lincoln: Vampire Hunter (2012),
The Truth About Emanuel (2013),
Knights of Badassdom (2013), and the
Roland Emmerich action thriller
White House Down (2013), where he played a villainous computer hacker. He then joined the cast of the
Netflix political thriller series
House of Cards, playing Gavin Orsay between 2014 and 2015. For this, Simpson was nominated on two occasions—alongside his co-stars—for the
Screen Actors Guild Award for Outstanding Performance by an Ensemble. In 2015, Simpson headlined a
Circle X Theatre production of
Trevor, a play written by
Nick Jones. His portrayal of the title character, a full-grown chimpanzee, was roundly praised, with
KCRW commenting, "You can't imagine the humanity that [Simpson] brings to
Trevor. Yes, it's a funny play and, yes, there's some 'monkey business' but Mr. Simpson's gift is restraint. Instead of playing for broad laughs, he plays Trevor's struggle for just that: an honest struggle".
2016–present: Westworld and career progression Simpson starred on the first season of
SundanceTV's dark comedy-drama
Hap and Leonard in 2016.
Den of Geek felt he displayed a "raw, manic energy" in his portrayal of Soldier—a psychotic drug dealer—that was "by turns infectious and terrifying", adding, "Seriously, [Simpson] is such a great bad guy". That same year, he appeared in a principal role on the debut season of
HBO's science fiction drama series
Westworld. His portrayal of William, a businessman who visits the titular
Wild West-themed amusement park and falls in love with one of its android inhabitants, was described as "spellbinding" by
Maureen Ryan of
Variety. Simpson was once again nominated alongside his co-stars for a Screen Actors Guild Award in 2017, while his work on the show's second season earned him an
Emmy Award nomination for
Outstanding Guest Actor in a Drama Series. Speaking of the pressure he felt being part of the hit show, Simpson said, "I think [a lot of us fear] that we won't be able to sleep at night if we deliver work that we're not happy with …
Westworld [is] a lot of responsibility. It's a huge show. There's so much money and publicity behind it … and [the creators] need us to show up and be as amazing as possible. So the fear is a factor, because you look over and you see Anthony fucking Hopkins … It leaves me wondering: "How the hell did I get here?"". In 2017, Simpson appeared as Walton, a lieutenant aboard the titular spaceship ("
USS Callister") in the opening episode of the fourth season of British anthology sci-fi series
Black Mirror. In their review,
Den of Geek called Simpson "one of [television's] best-kept secrets" and remarked that his performance "pops off the screen". His portrayal of Walton earned him a nomination for the
BAFTA Award for Best Supporting Actor the following year. Simpson starred as the real-life
Russell Poole in
Unsolved, a ten-part miniseries based on the 1990s murders of rappers
Tupac Shakur and
Biggie Smalls, which ran on the
USA Network between February and May 2018.
IndieWire praised the "elevated artistry" of Simpson's portrayal of the
LAPD detective, while
Vulture said in their review: Simpson's next roles were in
David Robert Mitchell's neo-noir black comedy
Under the Silver Lake (2018), the action thriller
Unhinged (2020), and the comedic crime drama
Breaking News in Yuba County (2021). He also headlined the ten-part
Epix series
Perpetual Grace, LTD in 2019, earning strong reviews for his portrayal of James Schaeler, an ex-firefighter embroiled in a conspiracy to scam a corrupt pastor; Darren Franich of
Entertainment Weekly described him as "an endearingly slippery protagonist, looking terrified, sad, amused, and exhausted all at once". Between April and July 2022, Simpson appeared as
CIA agent Spencer Clay in
Showtime's adaptation of
The Man Who Fell to Earth, which ran for a single season. In a mixed review of the series,
Vulture commented that it felt "unsure" of itself, but said of Simpson, "[he tears] into the material [in] amusing and engaging ways". In March that same year, he began playing Tom Andrews on the
Apple TV+ drama
Pachinko. Chronicling an immigrant Korean family across four generations, the series was universally praised. Simpson received positive notices for his portrayal of an alcoholic father in the 2023 coming-of-age film
The Starling Girl, an independent drama about
fundamentalist Christianity that Peter Debruge of
Variety felt was "refreshing" and "rigorously realistic". From 2023 to 2024, he voiced Doctor Royce Hemlock in seasons 2 and 3 of the
Disney+ animated series
Star Wars: The Bad Batch. Simpson wrote the upcoming horror comedy film
Slay. ==Personal life==