Writer Robert Schenkkan and actors Bryan Cranston (as Lyndon B. Johnson) and Anthony Mackie (as Martin Luther King) talk about their HBO film All the Way, which tells how Johnson, who became president of the United States after John F. Kennedy was assassinated, worked with King to pass the Civil Rights Act.

Was Johnson really down for this cause, or was this mere politics? Schenkkan: "He absolutely was down for this cause. But he was a very, very complicated fellow. One of my favourite descriptions of him is by [American political commentator] Bill Moyers, which was that, 'The 11 most interesting people I ever met was Lyndon Johnson.' And if you asked any of the individuals who have worked with him over the years, they would say something like, 'He was the kindest, cruelest, most selfless, selfish son of a bitch, warm-hearted, family, conniving, lying, decent man I ever met,' and they wouldn't be lying."

How did you research your role as Johnson? Cranston: "You do have an added responsibility, from an actor's standpoint, when you're playing a non-fictional character. Fortunately, for me, there was a plethora of source material and biographies by many different people which you become a student of. And you start cracking the books and talking to people. And there were many, many people alive and well who were willing to consult with me, and also with the story as a whole. Going down to the Johnson Library [at the University of Texas at Austin] a couple of times, they allowed me to be locked inside. I spent six, seven hours at a time by myself, looking."

Prior to this project, had you ever thought about playing King? Mackie: "I have been asked to play Martin Luther King and toyed with the idea a bunch. But I knew, as an individual and as an actor, I wasn't ready. And every script I had read, it wasn't the Martin Luther King that I knew. This was the first time I read a script where I felt like the writer actually got the essence of who Martin Luther King truly was. I feel like, if you look at somebody at that point in time, what he was undergoing in that movement, that man was not a pushover, and this was the first time I saw Martin Luther King as not being a pushover."

Did you manage to make it through many films of Johnson during your research? Schenkkan: "Those speeches of LBJ that we all remember, the ponderous, boring, put-you-to-sleep kind, that was not who the man was. He was, by all accounts, a charming, dead-on mimic, a great raconteur, the life of the party, charismatic. And the reason those speeches were that way was because he was so insecure about his own background; he worried that he didn't appear presidential, and so that mode of speaking that we all remember him from is completely an artifice. It used to drive his handlers crazy."

All the Way debuts on May 22 on HBO at 8am, with a repeat at 9pm.