I have no flexibility below the ass at all

Married to the daughter of comedienne/singer/actress Fanny Brice, producer Ray Stark wanted to make a musical about his mother-in-law. The musical he eventually made was Funny Girl, written by Isobel Lennart, with music by Jule Styne and lyrics by Bob Merrill. Cast as Fanny Brice was 22-year-old Barbra Streisand (whose Broadway debut in I Can Get It for You Wholesale had been a big success), while Sydney Chaplin was cast in the role of Brice's husband Nick Arnstein. Funny Girl first opened on Broadway in March 1964 and became a critical and commercial hit, prompting producer Stark a year later to start preparations for the film version.  

Sharif and Streisand in Funny Girl

Despite being a sensation on Broadway, Barbra Streisand had yet to make her screen debut. For the film, Columbia Pictures wanted a more established star and instead of Streisand chose Shirley MacLaine to play the lead. Ray Stark, however, wanted nobody but Barbra, even refusing to make the film if she was not cast. The studio eventually gave in to Stark's demand and Barbra got the part, giving a marvellous performance and earning the Oscar for Best Actress (in a tie with Katharine Hepburn for The Lion in Winter). 

For the role of gambler Nick Arnstein, several actors were considered, including Sean Connery, James Garner, David Janssen, Gregory Peck and Paul Newman. Composer Jule Styne wanted Frank Sinatra but Stark found him too old for the part. Also, Sinatra demanded too high a salary —$750,000 against Barbra's $200,000— ánd top billing (being the star of the film, Barbra refused to take second billing). In the end, Egyptian actor Omar Sharif, who had recently starred in the successful Doctor Zhivago (1965), was cast opposite Streisand at a salary of $50,000.

Like the original Broadway production, the film Funny Girl (1968), directed by veteran William Wyler, was a major success, both critically and commercially. It became the highest-grossing film of 1968 in the USA and received eight Academy Award nominations with, as said, Streisand winning the Oscar for Best Actress.

Barbra Streisand on the set of Funny Girl with (above) producer Ray Stark and (below) director William Wyler.

As mentioned above, Paul Newman was one of the actors approached to play Nick Arnstein. Newman, however, felt he was not right for the part and declined via this funny letter, written to Ray Stark and William Wyler in May 1967.

Source: Letters from Hollywood: Inside the Private World of Classic American Moviemaking (2019) by Rocky Lang and Barbara Hall. 

Although Paul Newman and Barbra Streisand never played in a film together, they were professionally linked in a different way. In June 1969, Newman and Streisand along with Sidney Poitier (all three pictured above) founded the production company First Artists in order to have more creative control over their own projects (one of the films the company produced was A Star Is Born (1976)). First Artists eventually ceased to operate in 1980 and was sold to Warner Brothers.