“Completely terrified.”That’s the way British television and screenwriter Peter Morgan characterizes his state of mind as he counted down the days before “Frost/Nixon” opened at Broadway’s Bernard J. Jacobs Theater. “If I could be a realist, this is somebody’s first play, and I’m just a kind of rookie,” he explains. “I didn’t write ‘Frost/Nixon,’ and say, ‘Here’s my debut for Broadway.’ The whole experience has accelerated out of control.” Pre-opening night jitters aside, Morgan has been having an explosively successful year. Blessed with the ability to breathe vivid dramatic life into historical figures, in addition to “Frost/Nixon,” he’s churned out the Oscar-nominated screenplay for “The Queen” (written in one week), co-authored the screen adaptation of “The Last King of Scotland” and scripted the HBO film “Longford.” When director Stephen Frears postponed the start of shooting of “The Queen,” in a fit of pique, Morgan turned to an idea that he says had been lurking in his mind for a decade — a play about David Frost’s 1977 series of interviews with ex-President Richard Nixon. The resulting “Frost/Nixon” proved an artistic and commercial success during its world premiere at London’s Donmar Warehouse and subsequent West End transfer. Unfazed by the acclaim, Morgan had a clinical reaction to a Wednesday matinee preview at the Jacobs Theater. “I’m intensely self-critical, and maybe I’m just being paranoid, but at the beginning, I said, ‘This is terrible.’ It wasn’t until about halfway through that I realized the audience wasn’t hating it. It sort of creeps up on you like the frog that you put in the water and turn the heat on and you don’t notice it’s cooking.” “Frost/Nixon” is scheduled to end its run on Aug. 13, and before the month is over Ron Howard starts filming Morgan’s screen adaptation of his play with Frank Langella and Michael Sheen repeating their stage roles as Nixon and Frost respectively.