David Tennant, who portrayed Barty Crouch Jr., is known for starring as the 10th Doctor.
Dumbledore actor Michael Gambon guest-starred on the 2010 "Doctor Who" Christmas special.
The same year that David Tennant played Barty Crouch Jr. in "Harry Potter and the Goblet of Fire," he made his debut as the 10th Doctor on "Doctor Who."
"Goblet of Fire," the fourth movie in the "HP" franchise, was released in November 2005.
Tennant played the Doctor for three series and returned to "Doctor Who" for later episodes. He's set to appear on the upcoming 14th series.
Michael Gambon, known for playing Albus Dumbledore in the "HP" films, guest-starred on the 2010 "Doctor Who" Christmas special.
Gambon played Kazran Sardick on the 2010 "Doctor Who" special, which was inspired by Charles Dickens' "A Christmas Carol."
Years later, during an appearance at the London Film and Comic Con, the actor said that he thought he "wasn't very good" on "Doctor Who" but had fun nonetheless.
In 2010, Helen McCrory reprised her "HP" role as Narcissa Malfoy for "Deathly Hallows: Part 1" and guest-starred on "Doctor Who."
McCrory portrayed Narcissa in the final three "HP" films: "Half-Blood Prince," "Deathly Hallows: Part 1," and "Deathly Hallows: Part 1."
On the fifth series of "Doctor Who," she played an alien named Rosanna Calvierri on an episode titled "The Vampires of Venice."
Bill Nighy played Rufus Scrimgeour in "Harry Potter and the Deathly Hallows: Part 1" and guest-starred as Dr. Black on the "Doctor Who" episode "Vincent and the Doctor."
In the penultimate "HP" movie, Nighy's character served as the Minister for Magic.
On the 2010 episode of "Doctor Who," Nighy played an expert on famed painter Vincent van Gogh.
Toby Jones voiced Dobby the elf in the "HP" franchise and played an antagonist on series five of "Doctor Who."
Jones played the Dream Lord on the "Doctor Who" episode titled "Amy's Choice."
Imelda Staunton joined the "Harry Potter" franchise as the villainous Dolores Umbridge in "Order of the Phoenix." She also did voice work for "Doctor Who."
Her unlikable "HP" character had an affinity for cats and the color pink.
On "Doctor Who," Staunton voiced the Interface on the series six episode titled "The Girl Who Waited."
Mark Williams played fathers in the "HP" movies and on "Doctor Who."
"HP" fans will recognize the actor for his role as patriarch Arthur Weasley. But he also played Brian, the father of Rory Williams, on two episodes of "Doctor Who."
David Bradley, who starred as Hogwarts' cranky caretaker named Argus Filch, portrayed more than one character on "Doctor Who."
On the 2012 episode "Dinosaurs on a Spaceship," Bradley portrayed a villain named Solomon. He later returned to the show as William Hartnell, the First Doctor.
Fellow "HP" actor John Hurt also played a Doctor on the sci-fi show.
Hurt, who portrayed the wandmaker named Ollivander in the "HP" movies, was introduced as the War Doctor at the end of the 2013 episode "The Name of the Doctor." He returned for the "Doctor Who" 50th anniversary special.
After portraying Barty Crouch Sr. in "Goblet of Fire," Roger Lloyd Pack guest-starred on two episodes of "Doctor Who."
He played a villainous businessman named John Lumic on the series two episodes titled "Rise of the Cybermen" and "The Age of Steel."
Adrian Rawlins, who had appearances as James Potter throughout the "HP" films, also had a role on a 2008 episode of "Doctor Who."
On the series four episode titled "Planet of the Ood," the actor played Dr. Ryder, the head of management at Ood Operations.
A few years after playing Hogwarts flying instructor Madam Hooch in the first "HP" movie, Zoë Wanamaker guest-starred on "Doctor Who."
On "Doctor Who," Wanamaker played Cassandra, a trampoline-like, stretched-out "pure human" who was in constant need of moisturizer.
Shirley Henderson, who had a memorable role as Moaning Myrtle in the "HP" movies, guest-starred on series two of "Doctor Who."
Henderson played the ghost in "Chamber of Secrets" and "Goblet of Fire."
She guest-starred as a character named Ursula Blake on the "Doctor Who" episode titled "Love & Monsters."
"Monty Python" star John Cleese had small roles in the "Harry Potter" films and "Doctor Who."
He played a Gryffindor ghost known as Nearly Headless Nick in "Sorcerer's Stone" and "Chamber of Secrets."
Cleese also had a cameo as an art gallery visitor on a 1979 episode of "Doctor Who."
In addition to playing "HP" characters, Warwick Davis also guest-starred on series seven of "Doctor Who."
In the "HP" franchise, Davis played Griphook and a Hogwarts professor named Filius Flitwick.
On the 2013 episode of "Doctor Who" titled "Nightmare in Silver," the actor played a museum curator named Porridge.
Long before having a minor role in "Goblet of Fire," Jeff Rawle appeared on "Doctor Who."
Rawle, who portrayed Cedric Diggory's dad named Amos, played a character known as Plantagenet on a few "Doctor Who" episodes released in 1984.
Elizabeth Spriggs, who played the portrait known as the Fat Lady in "Sorcerer's Stone," appeared on "Doctor Who" in the '80s.
Spriggs played a cannibal named Tabby on three episodes of "Doctor Who," released in 1987.
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