Reiner has said that it was the insecurity that loomed behind Johnson's larger-than-life persona that most interested him in pursuing the project.
A source told TMZ that Bieber met with Steinfeld after the MET Gala, which she attended solo.
The film follows Johnson from the time he loses the Democratic presidential nomination to John F.
Kennedy in 1960 and agrees to be the Massachusetts' senator's running mate, through his frustration at being sidelined as second fiddle and sudden transition into the Oval Office following Kennedy's assassination in 1963, and ultimately to his own legacy-building efforts to push the Civil Rights Act into law.
The newlyweds were also reportedly treated to a performance by singer Alanis Morissette.
Harrelson's latest film, Seven Pounds, which also stars Will Smith, opens in the UK in January.