Why was Grace Lee Whitney so highly billed in Star Trek TOS?

Question

Actress Grace Lee Whitney appeared in eight early episodes of Star Trek: The Original Series as Yeoman Janice Rand. The casts of these episodes (in the order that they are billed on-screen) are given below. Ignoring guest actors (given in italics), you can see that Yeoman Rand was usually listed fourth, after Kirk, Spock, and McCoy. She is always immediately after McCoy and ahead of the "regulars" Scotty, Sulu, and Uhura. Nurse Chapel is the only other "regular" billed higher than Yeoman Rand -- ahead of Dr. McCoy as well! -- but later in the series Chapel had a lower billing.

Why was Grace Lee Whitney so highly billed in the cast?

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"The Corbomite Maneuver"

• William Shatner as Capt. Kirk
• Leonard Nimoy as Mr. Spock
• Anthony Call as Dave Bailey
• Clint Howard as Balok
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• George Takei as Sulu
• James Doohan as Scott
• Nichelle Nichols as Uhura

"The Enemy Within"

• William Shatner as Capt. Kirk
• Leonard Nimoy as Mr. Spock
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• George Takei as Sulu
• James Doohan as Scott
• Edward Madden as Fisher
• Garland Thompson as Wilson
• Jim Goodwin as Farrell

"The Man Trap"

• William Shatner as Kirk
• Leonard Nimoy as Spock
• Jeanne Bal as Nancy Crater
• Alfred Ryder as Robert Crater
• DeForest Kelley as Leonard McCoy
• Grace Lee Whitney as Janice Rand
• George Takei as Sulu
• Nichelle Nichols as Uhura
• Bruce Watson as Green
• Michael Zaslow as Darnell
• Vince Howard as a Crewman
• Francine Pyne as Nancy III

"The Naked Time"

• William Shatner as Kirk
• Leonard Nimoy as Spock
• Stewart Moss as Tormolen
• Majel Barrett as Christine
• Bruce Hyde as Riley
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• George Takei as Sulu
• James Doohan as Scott
• Nichelle Nichols as Uhura
• William Knight as amorous crewman
• John Bellah as a laughing crewman

"Charlie X"

• William Shatner as Kirk
• Leonard Nimoy as Spock
• Robert Walker as Charles Evans
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• Nichelle Nichols as Uhura
• Charles J. Stewart as Captain Ramart
• Dallas Mitchell as Nellis
• Don Eitner as the Navigator
• Patricia McNulty as Tina Lawton
• John Bellah as Crewman I
• Garland Thompson as Crewman II
• Abraham Sofaer as "The Thasian"

"Balance of Terror"

• William Shatner as Capt. Kirk
• Leonard Nimoy as Mr. Spock
• Mark Lenard as a Romulan Commander
• Paul Comi as Stiles
• Lawrence Montaigne as Decius
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• George Takei as Sulu
• James Doohan as Scott
• Nichelle Nichols as Uhura
• Stephen Mines as Tomlinson
• Barbara Baldavin as Angela
• Garry Walberg as Hansen

"Miri"

• William Shatner as Capt. Kirk
• Leonard Nimoy as Mr. Spock
• Kim Darby as Miri
• Michael J. Pollard as Jahn
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• Keith Taylor as a little boy (miscredited as playing "Jahn's Friend")
• Ed McCready as a boy creature
• Kellie Flanagan as a blonde girl
• Steven McEveety as a redheaded boy
• David Ross as Security Guard #1
• Jim Goodwin as Farrell
• John Megna as Jahn's Friend (miscredited as playing "Little Boy")

"The Conscience of the King"

• William Shatner as Capt. Kirk
• Leonard Nimoy as Mr. Spock
• Arnold Moss as Karidian
• Barbara Anderson as Lenore
• DeForest Kelley as Dr. McCoy
• Grace Lee Whitney as Yeoman Rand
• Nichelle Nichols as Uhura
• William Sargent as Dr. Leighton
• Natalie Norwick as Martha Leighton
• David-Troy as Larry Matson
• Karl Bruck as King Duncan
• Marc Adams as Prince Hamlet

Answer 1

She was originally intended to have a much larger role in the show, but was slowly written out when plans changed. She was released from her contract entirely after eight episodes.

The Wikipedia article for Grace Lee Whitney has all the details (sourced from her own memoir.)

Answer 2

Whitney was a known entity to Roddenberry having worked with him before and had worked quite extensively in TV before.

Based on salary, her "billing" seems correct.

Grace Lee Whitney came on board as Janice Rand, the Captain’s Yeoman. She was 35. A former stage dancer with more than three dozen credits in television and film, she had recently played a lead role in “Controlled Experiment,” the only comedy episode produced for The Outer Limits. Whitney first met Roddenberry on The Lieutenant. Charmed by the pretty blonde, he cast her as the female lead in “Police Story.” And that led to Star Trek.

^{These Are The Voyages - TOS Season 1 by Marc Cushman}

Over and above that, excepting Shatner and Nimoy, Whitney had one of the better Star Trek contracts with Desilu.

She was contracted/guaranteed for 7 of the 16 episodes at $750 per episode.

Only DeForrest Kelley had better (again, Shatner & Nimoy excepted) at 7 episodes at $850 per.

In comparison, James Doohan was contracted for only 5 episodes (albeit at $850 per episode) and George Takei (7 episodes at $600 per episode)

^{Source as before}