“Diane” is a 1956 American historical film drama about the life of Diane de Poitiers, produced by Metro-Goldwyn-Mayer, directed by David Miller, and produced by Edwin H. Knopf from a screenplay by Christopher Isherwood based on a story by John Erskine.
The music score was composed by Miklós Rózsa, and Robert H. Planck was the cinematographer, who filmed in CinemaScope and Eastmancolor. The exceptionally lavish costumes were designed by Walter Plunkett. Though Lana complained that they were terribly uncomfortable. She never made a costume picture again.

The film stars Lana Turner, Pedro Armendáriz, Roger Moore, and Marisa Pavan, and features Sir Cedric Hardwicke, Torin Thatcher, Taina Elg, John Lupton, Henry Daniell, Melville Cooper and an early film appearance by Stuart Whitman.
It was Turner’s last film under her longtime MGM contract and thus marked another stage in the decline of the studio star system.
The action is set in 16th-century France.
Diane de Poitiers (Lana Turner) becomes the mistress of Prince Henri (Roger Moore), second in line to the throne.
Their liaison continues through Henri’s arranged marriage to the Italian Catherine de’ Medici (Marisa Pavan).
Unknown to Catherine, her Medici relations arrange the death of the Dauphin and Henri’s ascent to the throne as King Henry II.
The antagonism of the two women, abetted by Medici scheming, eventually results in the death of Henri.
Catherine, now ruling as regent for her three young sons, banishes Diane but spares her rival’s life in a gesture of mutual respect.

Lana Turner plays Diane de Poitiers and said this about the role of Diane: “Diane was an interesting woman”, one who used her charm intelligently”. Actually she was a forerunner of today’s modern woman; she was Europe’s first outdoor girl, a health fan and an advocate of the cold bath. She wasn’t afraid to use her head, but was never caught with her brains showing.”

Lana worked hard on this movie. She had to learn to ride side-saddle and together with her daugther Cheryl, who was 12 at the time, they went to Culver City and learned it together. They were horseback riders, adept at English and Western, but they found that riding side-saddle was not so easy. Their backs would get sore being twisted, but they had great laughs. 
Fencing was another skill she had to acquire. She learned it easily because of her rhythm and dancing abilities. She was proud that she was able to do all the fencing in the film and no doubles were needed.

Released in January 1956, “Diane” proved to be a signifiant film in Lana’s career. One month later, while budget-conscious MGM was in the midst of disbanding its expensive star system, Lana terminated her 18-year association with the studio. Although frankly sentimental about the studio that had been her home since 1938, her departure from Metro was a judicious move. Her next film (“Peyton Place“) and her first film as a free-lance actress marks another turning point in her career and ultimately brought her an Oscar nomination.

I love “Diane”, the story, the costumes, everything about it. 😀

Trailer:

Clip:

You can watch the film here.

Lana Turner did a lot of movies. She really loved being a moviestar.
Here are the most important films in which she starred. Just click the links for more info, photos and videos.

20 April 1937: A Star Is Born
9 Oct. 1937: They Won’t Forget
24 Oct. 1937: The Great Garrick
1 Jan. 1938: The Adventures of Marco Polo
22 July 1938: Love Finds Andy Hardy
12 Aug. 1938: Rich Man, Poor Girl
9 Dec. 1938: Dramatic School
28 April 1939: Calling Dr. Kildare
18 Aug. 1939: These Glamour Girls
29 Sept. 1939: Dancing Co-Ed
19 April 1940: Two Girls on Broadway
19 July 1940: We Who Are Young
25 April 1941: Ziegfeld Girl
12 Aug. 1941: Dr. Jekyll and Mr. Hyde
2 Oct. 1941: Honky Tonk
18 Febr. 1942: Johnny Eager
27 Aug. 1942: Somewhere I’ll Find You
1 April 1943: Slightly Dangerous
19 Aug. 1943: Du Barry Was A Lady
13 Nov. 1943: The Youngest Profession
23 Aug. 1944: Marriage Is a Private Affair
8 March 1945: Keep Your Powder Dry
1 Oct. 1945: Week-End At The Waldorf
2 May 1946: The Postman Always Rings Twice
5 Nov. 1947 : Green Dolphin Street
1 Nov. 1947: Cass Timberlane
29 April 1948: Homecoming
20 Oct. 1948: The Three Musketeers
1 Sept. 1950: A Life Of Her Own
2 March 1951: Mr. Imperium
5 Sept. 1952: The Merry Widow
25 Dec. 1952: The Bad And The Beautiful
25 Aug. 1953: Latin Lovers
1 May 1954: Flame And The Flesh
7 Sept. 1954: Betrayed
13 May 1955: The Prodigal
4 June 1955: Sea Chase
14 Dec. 1955: The Rains Of Ranchipur
12 Jan. 1956: Diane
13 Dec. 1957: Peyton Place
30 Jan. 1958: The Lady Takes A Flyer
2 May 1958: Another Time, Another Place
30 April 1959: Imitation Of Life
23 June 1960: Portrait In Black
19 July 1961: By Love Possessed
2 Nov. 1961: Bachelor In Paradise
25 Dec. 1962: Who’s Got The Action?
24 Febr. 1965: Love Has Many Faces
27 April 1966: Madame X
30 April 1969: The Big Cube
7 Nov. 1974: Persecution
27 Oct. 1976: Bittersweet Love
31 Oct. 1980: Witches Brew
21 May 1982: Dead Men Don’t Wear Plaid

Sources: Wikipedia, IMDB, Rotten Tomatoes, Lana’s biography “Lana, the Lady, The Legend, The Truth“, “The Films Of Lana Turner” by Lou Valentino and “Lana Turner, The Memories, The Myths and The Movies” by Cheryl Crane.