Theme: Combination of the Two - The first part of the unifier is placed behind the first part of the starred entries to form an unrelated combination.
56A. Process for selecting theatrical performers, and a hint to the first word of the answers to starred clues : CASTING CALL
20A. *Manhattan site of Strawberry Fields : CENTRAL PARK. Central Casting, where all the movie extras come from.
39A. *Prepare for printing : TYPESET. Typecasting, what some actors suffer.
11D. *Untrustworthy, as a business : FLY-BY-NIGHT. Fly Casting, a type of fishing. Clip
29D. *Word processor error finder : SPELL CHECK. Spell Casting, just one of the classes at Hogwarts.
Argyle here. Every letter but the 'J'. Putting the uni-word in the middle of the answer makes it harder to spot. I felt the level of fill was slightly uneven but doable. Some room for cornerites to add their own spin. Have at it.
Across:
1. Halloween costume part : MASK
5. 640 acres: Abbr. : SQ MI. (square mile)
9. Longtime employee : LIFER
14. Death Valley's is the lowest in North Amer. : ELEV. 282 feet below sea level.
15. Mammoth feature : TUSK. Woolly mammoth.
16. China's Zhou __ : ENLAI
17. "Dirty Jobs" host Mike : ROWE. ...but somebody has to do it.
18. County Kerry's isle : EIRE
19. Deep chasm : ABYSS
23. "So long" : "BYE"
24. Young horse : COLT
25. One from Nairobi : KENYAN
27. Ultimate conclusion : UPSHOT. 1530's; originally, the final shot in an archery match, hence the figurative sense of "result, issue, conclusion". ~ Online Etymology Dictionary
30. Made of oak, e.g. : WOODEN
32. Small swallow : SIP
33. Pumps or clogs : SHOES
35. Thin piece of change : DIME
38. __ out a living : EKE
42. Guy's partner : GAL
43. Bank (on) : RELY
45. Glue in a hobbyist's kit : EPOXY
46. "Let me think ... " : "HMM..."
47. Utter madness : LUNACY
50. Michelangelo masterpieces : PIETAs
52. Tallied, with "up" : RACKED
54. Group after boomers : GEN X
55. "How relaxing!" : "AAH!"
62. Bit of luck : BREAK
64. Place for koi : POND
65. Prefix with distant : EQUI
66. Italian ball game : BOCCE
67. Woodworking tool : ADZE
68. Put on a pouty face : SULK
69. Filled (in), as a comic strip : INKED
70. Frog's kiddie-lit friend : TOAD
71. Yard event : SALE
Down:
1. Bygone Ford division, for short : MERC. A 71-year run ended in 2010.
2. Sunburn soother : ALOE
3. Stitched up : SEWN
4. Complain : KVETCH
5. Furtive : STEALTHY
6. Comforter to get comfy in : QUILT
7. Car sticker abbr. : MSRP. (manufacturer's suggested retail price)
8. Swedish furniture chain : IKEA
9. Didn't hold water : LEAKED
10. Having five sharps, musically : IN B
12. Course that's good for one's GPA : EASY 'A'
13. Ascended : RISEN
21. Barnyard perch : ROOST
22. MGM rival : RKO. (Metro-Goldwyn-Mayer/Radio-Keith-Orpheum)
26. Homer's nice neighbor : NED Flanders.
27. Password creator : USER
28. High-speed highway : PIKE
30. Teary-eyed : WEEPY
31. Bone, in Italian : OSSO
34. Oil gp. with 12 member nations : OPEC. (Organization of the Petroleum Exporting Countries)
36. Papa's partner : MAMA
37. Stately shade trees : ELMS
40. Paid out : EXPENDED
41. Making, as a knot : TYING. Tying a fly.
44. Big laugh : YUK
48. Did some smooching : NECKED
49. Aquafresh tube letters : ADA. (American Dental Association)
51. Overabundance : EXCESS
52. Talmud expert : RABBI
53. Brother of Moses : AARON
54. Tokyo shopping district : GINZA
57. Petty quarrel : SPAT
58. Chore list heading : TO DO
59. Greenish-blue : AQUA. A little bigger clue/answer dupe (w/ 49D) than yesterday.
60. Temporary calm : LULL
61. Similar to : LIKE
63. Unreturnable serve : ACE. Tennis.
Argyle