Denzel Washington is younger than Mandela was during this chapter of his life, but with the right makeup he would certainly fit the part. Washington is a two-time Academy Award winner and has starred in biopics before—he starred as the title character in Spike Lee's Malcolm X and played Rubin "Hurricane" Carter in Norman Jewison's Hurricane. In this film, he would play a version of Mandela who is focused on humanitarianism.
Mandela's second wife Winnie stood by him through his long years of imprisonment into the first years of his presidency, and the two remained close until the end of his life. Winnie found herself a well known public figure in her own right while her husband was in prison, serving as his voice for the people of South Africa. Viola Davis has had a long and successful career in TV and is currently playing the lead role in How to Get Away with Murder, which earned her an Emmy award and two Golden Globe nominations among other accolades.
F.W. de Klerk was the last State President of South Africa under apartheid, and one of Nelson Mandela's deputy presidents when Mandela succeeded him in office. The two share the 1993 Nobel Peace Prize for their partnership in dismantling apartheid. Following his most famous role as Hank Schrader on Breaking Bad, Dean Norris was a recurring cast member on Under the Dome until late last year and fans of these two shows are likely waiting to see what he will do next. The firm and outspoken De Klerk would be a familiar role for the Breaking Bad star.
Graca Machel was Mandela's wife in the later years of his life until his death, and is an important politician herself. She is one of the twelve members of human rights group "The Elders" (others include Jimmy Carter and Desmond Tutu), which was founded by Nelson Mandela to help tackle some of the world's biggest conflicts. While she is best known as a TV producer and host Oprah Winfrey does occasionally take act in films, such as in Ava DuVernay's Selma.
Mbeki was deputy president under Mandela and succeeded him as President of South Africa in 1999. He was an important political ally to Mandela during this phase of his life, and continued Mandela's work in reversing the lasting effects of apartheid. Danny Glover is 69 years old, around the same age Mbeki was during his term, and is still very much a working actor with an impressive 13 projects currently in various stages of production.
As a white Afrikaner Zelda la Grange was a part of the very same oppressive system Mandela spent his life dismantling, and the fact that Mandela trusted her to be his personal assistant speaks to his capacity for forgiveness and compassion. Reese Witherspoon bears considerable resemblance to la Grange, and while some still fondly remember Elle Woods in Legally Blonde, this actress is capable of playing much more substatial roles. Her dramatic roles in films like Wild and Walk the Line got the attention of critics and Academy voters and earned her the Oscar for Best Actress in 2006.