Question: What careers doMichael Ritchie and Derek Estlin Purvis have in common? 

Context articles: Airtight (film)
List of EastEnders characters (2001)
Derek Estlin Purvis
Stray (band)
Derekh Eretz Zutta
Shaking Ray Levis
Fitness professional
Michael Ritchie (film director)
What Goes Up (short story)
Doctor Who (series 9)

Airtight is a 2014 American drama thriller film written, produced and directed by Derek Estlin Purvis.
 A psychological thriller set in a NY mansion and an homage to Alfred Hitchcock, "Airtight" is based on the true events that inspired a play in the 1920s, an adaptation by Hitchcock in the 1940s and this feature film from 2013.
The following is a list of characters that first appeared in the BBC soap opera "EastEnders" in 2001, by order of appearance.
 All characters were introduced by the show's executive producer, John Yorke.
 The first character to be introduced was Jill Marsden, a detective chief inspector investigating who shot Phil Mitchell (Steve McFadden).
 April saw the introductions of: Ritchie Stringer (Gareth Hunt), an associate of Phil, Paul Trueman (Gary Beadle), the son of Audrey Trueman (Corinne Skinner-Carter) and brother of Anthony Trueman (Nicholas Bailey), and Donna Andrews (Alison Senior; Paula Jennings), the mistress of Trevor Morgan (Alex Ferns).
 Harry Slater (Michael Elphick), the brother of Charlie Slater (Derek Martin), and Gary Bolton (Bruce Byron), the father of Robbie Jackson (Dean Gaffney), made their debuts in May, whereas Margaret Walker (Susan George), a love interest for Terry Raymond (Gavin Richards), and Angel Hudson (Goldie), a gangster, made their first appearances in June.
Derek Estlin Purvis (born November 27, 1972) is an American film director, producer, executive producer, screenwriter, and entrepreneur.
 He has financed, produced or directed 13 films of various genres.
 He is a founder and creator of the Application Movie Pitch Pro.
Stray is an English hard rock band formed in 1966.
 Vocalist Steve Gadd (born Stephen Gadd, 27 April 1952, Shepherd's Bush, West London), guitarist Del Bromham (born Derek Roy Bromham, 25 November 1951, Acton, West London), bass player Gary Giles (born Gary Stephen Giles, 23 February 1952, North Kensington, West London) and drummer Steve Crutchley (born c 1952) formed the band whilst all were attending the Christopher Wren School in London.
 Richard "Ritchie" Cole (born 10 November 1951, Shepherd's Bush, West London) replaced Crutchley in 1968.
 They signed to Transatlantic Records in January 1970.
Derekh Eretz Zutta (Hebrew: מסכת דרך ארץ זוטא) is a non-canonical tractate of the Babylonian Talmud.
 The name is misleading in more than one respect; the word "zuṭa" (small) would seem to indicate that it is a shorter version of the treatise "Derek Ereẓ Rabbah," which is not the case, the two having little in common.
 "Derek Ereẓ," moreover, is a very unsuitable name for a collection of ethical teachings such as form the substance of the treatise.
 Even Rashi, however (Ber.
 4a), knows the treatise under this name, calling it "Masseket Derek Ereẓ," while the Tosafists likewise call it "Hilkot Derek Ereẓ" (Bek.
 44b).
 The designation "zuṭa" is probably of later.
The Shaking Ray Levis is an ongoing collaboration of musicians with a common interest in free improvisation.
 The project was conceived and led by the Chattanooga, Tennessee-based team of Dennis Palmer and Bob Stagner.
 They use storytelling, synthesizers, samplers and percussion to achieve their distinctive sound.
 They are the first American group to have recorded for Incus Records, the record label of British free improvisational guitarist Derek Bailey.
 Additionally, they have performed and recorded with Borbetomagus, John Zorn, David Greenberger, Killick Erik Hinds, Fred Frith, Min Tanaka, Amy Denio, and Derek Bailey, as well as with many other critically acclaimed artists.
A fitness professional is a professional in the field of fitness and exercise, most often instruction (fitness instructor), including aerobics and yoga instructors and authors of fitness instruction books or manuals.
 Fitness topics may also include nutrition, weight-loss, and self-help.
 Fitness careers are distinguished from exercise science careers such as athletic training, however the various types of fitness certifications have more and more in common: the, "distinctions...have become blurred, with more similarities than differences given the common background that all fitness professionals must possess."
Michael Brunswick Ritchie (November 28, 1938 – April 16, 2001) was an American film director of films with comical or satirical leanings, such as "The Candidate" and "Smile".
 He scored commercial successes directing sports films like "The Bad News Bears" and Chevy Chase's "Fletch" comedies.
"What Goes Up" is a science fiction short story by Arthur C. Clarke, first published in 1956, and later anthologized in Tales from the White Hart.
 Like the rest of the collection, it is a frame story set in the fictional pub "White Hart", where Harry Purvis narrates the secondary tale.
 The title is a reference to the common phrase "What goes up must come down."
The ninth series of the British science fiction television programme "Doctor Who" premiered on 19 September 2015 with "The Magician's Apprentice" and concluded on 5 December 2015 with "Hell Bent".
 The series was led by head writer and executive producer Steven Moffat, alongside executive producer Brian Minchin.
 Nikki Wilson, Peter Bennett, and Derek Ritchie served as producers.
 The series is the ninth to air following the programme's revival in 2005, and is the thirty-fifth season overall.