Ronald Reagan need not apply for a job as ABC TV anchorman. Av Westin wouldn't hire him. According to Mr. Westin - vice-president for program development at ABC News, executive producer of ''20/20,'' executive in charge of ''The Last Word,'' and author of the just-published book ''Newswatch: How TV Decides the News'' (Simon & Schuster, $15.95) - ''Ronald Reagan is a great communicator, but the bottom line for an anchor is an ability to conceal biases. Mr. Reagan is obviously a man of great political passion, and I suspect that he has been doing commentary for so long that he would not be able to overcome his biases. Now, if he'd like a job as a commentator. . . .''
We are talking in a borrowed office (belonging to ABC News president Roone Arledge) at the ABC News headquarters. Mr. Westin's book, while filled with a behind-the-scenes view of the nation's top newsmen, also contains valuable information about the preparation of America's news programs. The book's main thesis is that television news has finally come of age although it is still the most rapidly changing information medium.
Av Westin is one of the new breed of television news people. From the start he worked and trained in television news rather than print news - and he is proud of that background. Now, as one of the top executives in network television news, he has been praised by some for being innovative and condemned by others for what seems to be an entertainment-oriented approach to news programs, particularly in the two he now supervises, ''20/20'' and ''The Last Word.''
Both shows try to inject a kind of disco beat into the solemn atmosphere of the newsroom. ''20/20'' (Thursdays, 10-11 p.m.) has won a more than respectable packet of Emmies and seems to be secure in the schedule, despite its uneven ratings record. ''The Last Word'' (nightly, midnight-1 a.m., after Ted Koppel's ''Nightline'') is still in the throes of experiment and reorganization. It comes across as more a radio show than a TV show, more a slight news-oriented entertainment than a serious news show. Mr. Westin indicates there are still many format changes to come.
Mr. Westin, who was actively involved in the major changes which brought ABC's ''World News Tonight'' from third place to a strong second place, behind CBS, tells in his book about the attempts at various times by ABC to hire such anchormen as Dan Rather, Tom Brokaw, Robert MacNeil, and Bill Moyers. What does he feel are the major attributes of a good anchorman?
''Although the anchor is of principal importance, it is only the gloss on the package. The package must contain the news people want or they are going to turn away, no matter who the anchor is.
''Credibility is what makes an anchor a star. And that comes from being on the scene consistently through a number of major news events so the audience has shared a good experience with him. Personality on air that encourages trust is also important.''
Does the fact that CBS News seems to have had an upturn in viewership on the nights Dan Rather wore a sweater impress Av Westin?
He laughs and shakes head just a bit sadly. ''Well, it is fine to see that the anchor is at least being made into a human being. Dan could wear four sweaters and mittens, too, but if he were not delivering the news, it wouldn't make any difference.''
What does Av Westin believe to be the next step forward for TV news?
''In the area of technology, I predict something which will help us to find a way to close the loop with the audience. There has got to be some way which will enable us to deal more directly with viewers so that they don't just sit there passively. Maybe it will be something like the QUBE (two-way television) transactional system or some other method, but we will have to figure out a way to get audience reactions and receive audience questions more directly. It is bound to happen soon.
''We are also getting into portability to an unbelievable degree. Cameras will soon be no bigger than a miniature recorder, and will deliver pictures and sound back from the field with equipment no bigger than a suitcase. It will enable us to go more places more rapidly; TV reporters will be able to operate with the ease of a print reporter. No more big cameras and special lights.''
Why does Mr. Westin believe there is such a sudden rush to expand the TV news at all hours? All of the networks are moving into late-night and early-morning news, and there are now two 24-hour news cable services.
''All of us in news have recognized that substantial numbers of people want information when they want it, not when we say they have to have it. So, we are being forced to give them news at a series of different times and places. I think that is good.''
Despite his enthusiasm for TV news, Mr. Westin is upset by surveys that indicate a large percentage of the American public depends solely upon TV for its news.
''If it is really true that 65 percent or so of all Americans get most or all of their news from us, those individuals are woefully underinformed. TV news starts by eliminating rather than including. Of course we are there and we provide a service, but I wish people would read newspapers and magazines to supplement what they get on TV.
''The truth is we are an entity now which does some things superbly, some things only passably well. What we can do best is get the story to the public fast with a superb combination of picture, natural sound, and narration. What we don't do better than newspapers and magazines is the reflective pieces.
''We've cleaned our house a lot; happy talk is almost over. The three networks are in fierce competition for excellence, and that tends to make us all better. Yes, the TV news business has matured. . . . It is about time it was recognized by everybody that what network television news does well, it does superbly.''