Wild about movies has an extensive article about the behind the scenes of Frost/Nixon. You can read the full article here.

Matthew Macfadyen had this to say in the article:

“John Birt was a producer at the time for LWT,”says Macfadyen, who met the real Birt for lunch prior to filming, as well as at the end of the shoot when Birt visited the Los Angeles set. “He’d worked with David Frost before, very happily and successfully, and Frost sort of poached him from this company for whom he’d just gone to work. Birt was actually my age, 32, at the time of the interviews, but he was already an incredibly successful producer. I think Birt had doubts about Frost’s ability to pull off these interviews.”

Frank Langella has done an interview with the Times about how filming Frost/Nixon was different from the play.  You can read the entire interview here.

A small excerpt:

He and Michael Sheen, who revisits his stage role as Frost, had to forget “all the things we did to play it to the back row in the theatre” – all the exaggerated movements and projected voices. “The camera was up your nose, it can pick up the tiniest tic on the face, the slightest raised eyebrow. The camera gives you the freedom to go much deeper.”

Howard was “relentless” in the filming, Langella reveals. “We rarely did less than 28 takes of any scene. He asked me to write down what the emotional highlight of each scene was for my character. This was wonderful. There were no long discussions. It was very useful shorthand. When we filmed the phone-call scene, on stage I would just say it. For the film we figured out how I could walk, look out of the window, while using the speaker-phone.”