(from: Buzz Feed)

• David Tennant

  

  Tennant played Barty Crouch Jr. in Harry Potter and the Goblet of Fire and the Tenth Doctor.

• Michael Gambon

  

  Gambon played Albus Dumbledore in the Harry Potter films, and both Kazran & Elliot Sardick in the Doctor Who episode A Christmas Carol.

• Helen McCrory

  

  McCrory played Narcissa Malfoy in the Harry Potter films, and a Saturnynian queen named Rosanna Calvierri in the Doctor Who epsiode The Vampires of Venice.

• John Cleese

  

  Cleese played Nearly Headless Nick in the Harry Potter films and has a small cameo in a Doctor Who episode titled City of Death.

• Zoë Wanamaker

  

  Wanamaker played Rolanda Hooch in the Harry Potter films, and the last living human named Lady Cassandra in several episodes of Doctor Who.

• Jim Broadbent

  

  Broadbent played Horace Slughorn in the Harry Potter films, and played a possible future version of the eight doctor in the episode The Curse of Fatal Death.

• Toby Jones

  

  Jones voiced the character of Dobby the House elf in the Harry Potter films, and played the Dream Lord in the Doctor Who episode Amy’s Choice.

• Bill Nighy

  

  Nighy played Rufus Scrimgeour in the Harry Potter films, and has a cameo as Vincent van Gogh expert in the Doctor Who episode Vincent and the Doctor (which will make you cry if you ever watch it).

• Roger Lloyd Pack

  

  Pack played Barty Crouch Sr. in the Harry Potter films, and played the creator of the Cybermen, John Lumic, in Doctor Who.

• Shirley Henderson

  

  Henderson played Moaning Myrtle in the Harry Potter films, and played Ursula Blake in the Doctor Who episode Love & Monsters, where she had her face fused with a slab of pavement by The Doctor.

• Adrian Rawlins

  

  Rawlins played Harry Potter’s father, James, in the films and played the leader of the Ood rebellion in the Doctor Who episode Planet of the Ood.

• David Bradley

  

  Bradley played Argus Filch in the Harry Potter films, and voiced the vulture humanoids known as the Shansheeth in Doctor Who.

• Derek Deadman

  

  Deadman played Tom the Innkeeper in Harry Potter and the Sorcerer’s Stone (he was later replaced by Jim Tavaré) and played a Commander in the Sontaran Special Space Service named Stor in a 1978 episode of Doctor Who.

• Elizabeth Spriggs

  

  Spriggs played the portrait of the fat lady guarding Gryffindor Tower in the Harry Potter films, and appeared in a 1987 episode of Doctor Who (where she is strangled by a cleaning device controlled by aliens.)

• Jeff Rawle

  

  Rawle plays Amos Diggory in Harry Potter and the Goblet of Fire (father to Cedric Diggory), and played Plantagenet in a 1984 episode of Doctor Who titled Frontios.

• Jessica Hynes

  

  Hynes provided the voice for Mafilda Hopkirk in the Harry Potter films before the character appeared inHarry Potter and the Order of the Phoenix, where she was played by Sophie Thompson. In Doctor Who, she played three separate related characters who The Doctor meets at various points in time.

• Jimmy Gardner

  

  Gardner played the driver of the Knight Bus in Harry Potter and the Prisoner of Azkaban and played an alien named Idmon in a 1978 episode of Doctor Who.

• John Atterbury

  

  Atterbury played the portraits of Phineas Nigellus Black in the Harry Potter films, and did some costume acting for several episodes of Doctor Who in the 1960s.

• Julian Glover

  

  Glover provided the voice for Aragog in the Harry Potter films, and played both Richard the Lionheart in a 1965 episode of Doctor Who as well as an alien named Scaroth in a 1979 episode.

• Terence Bayler

  

  Bayler plays the ghost and portrait of the Bloody Baron in the Harry Potter films, and appears as an alien named Yendom in an episode of Doctor Who from 1966.

• Other Connections:

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  The actor that plays Dudley in the Harry Potter films, Harry Melling, is actually the grandson of Patrick Troughton, who played the second doctor.

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  Alfie Enoch, who plays Dean Thomas in the Harry Potter films is the son of William Russell, who played Ian Chesterton, one of The Doctor’s first companions.