Joss Whedon has a pack of actors… just looking at the cast of Much Ado About Nothing, there are only two or three who I don’t recognize as having been in Buffy, Angel, Dollhouse, or Firefly. TV, of course, but if you add Cabin in the Woods & The Avengers, a bunch of the characters in this movie were in those, too.
Clint Howard seems to be fairly successful getting parts in Ron Howard’s movies, for some reason.
His Mom and his Dad (Rance Howard) have gotten a few roles in Ron’s movies, too! His mother played Jim Lovell’s Mom in Apollo 13, and Rance appeared beside her as a clergyman offering consolation.
Frank Darabont seems to have a stable of actors:
Jeffrey DeMunn
Laurie Holden
Dee Croxton
Brian Libby
Sam Witwer
Melissa McBride
William Sadler
I know there are others.
Bruce Willis and his friend Michael Clarke Duncan appeared together in The Whole Nine Yards, Breakfast of Champions, Armageddon and Sin City.
Willis also called the aforementioned Frank Darabont and told him “I have the perfect John Coffey for you” when he heard Darabont was making The Green Mile.
That was the artifact from the old studio system. Actors were signed by a studio and were assigned to films by the studio. If they wanted to work for another studio, they had to get permission. One stumbling block to the production of Gone With the Wind was that Selznick had to borrow Gable (the public’s choice for the role) from MGM. Gable made It Happened One Night because MGM lent him out to Columbia – some sources say it was “punishment,” though it seems that MGM just didn’t have any roles for him. Davis couldn’t do It Happened One Night because Jack Warner refused to lend her.
When the studio system died in the 50s, this no longer occurred, since the actors didn’t have contracts except on a picture-by-picture basis.
In Divergent, Shailene Woodley and Ansel Elgort are brother and sister.
In Fault in Our Stars, they are a dating couple.
Icky. Not really at all since they are not related, but still kind of odd casting.
::Slight hijack:: Rance and Clint were both in “Gentle Ben” on TV in the late '60s (yes, I’m old).
Tom Hanks and Sally Field were dating in Punchline, and then in Forrest Gump, she played his mom.
Aaron Taylor Johnson and Elizabeth Olsen played a husband & wife (with a son) in the most recent “Godzilla” remake. They are set to star as brother & sister superheroes Quicksilver & the Scarlet Witch in the upcoming “Avengers 2” flick.
Judd Apatow has a large group of actors that appear in his movies and other projects, detailed in this handy wikipedia chart.
Good calls on all three- Clint has made multiple movies with John Vernon, Harry Guardino, Geoffrey Lewis, Pat Hingle, Bill McKinney, and Sondra Locke.
Mel Brooks had Gene Wilder, Madalyn Kahn, Dom DeLuise, Marty Feldman, Cloris Leachman, and Harvey Korman.
Guest has Michael McKean, Harry Shearer, Jane Lynch, Paul Dooley, Eugene Levy, Catherine O’Hara, Fred Willard, Parker Posey and Bob Balaban.
James Garner and Stuart Margolin.
Johnny Galecki, Sara Gilbert, and Laurie Metcalf.
Bea Arthur and Rue McClanahan.
Ernest Borgnine and Robert Ryan appeared together in Bad Day at Black Rock, The Dirty Dozen, and ***The Wild Bunch, ***
Speaking of whom, Shailene Woodley and Miles Teller appear together in both Divergent and The Spectacular Now.
Ernest Borgnine and Lee Marvin were together in Violent Saturday, *** The Stranger Wore a Gun, *** Bad Day at Black Rock, The Dirty Dozen, and*** Emperor of the North Pole.***
<snip>
I know you can’t name everyone, but why not Albert Popwell? “I gots to know.” 
There’s usually 3 or 4 Cusacks in every John Cusack movie, either in front of or behind the cameras. Joan, it seems, almost always.
Of course! He was also Big Ed Mustafa in The Enforcer, and Horace in Sudden Impact!
AND the Drano-killer pimp in Magnum Force. (How about those exhaust pipes on his Pimp-mobile? He must have sold that car to the Duke of New York.)