Boston — During a strike of the American Federation of Television and Radio Artists in 1967, Arnold Zenker became a network TV personality. He substituted for three weeks for Walter Cronkite on ''CBS Evening News.'' Viewers responded with fan mail lauding his performance. When Cronkite returned, he opened the news with ''Good evening. This is Walter Cronkite sitting in for Arnold Zenker.''
Last month Arnold Zenker made national TV again. This time he was interviewed by Mike Wallace on ''60 Minutes.'' The questions were about his business, that of helping clients communicate effectively in the public spotlight. When the show was over, Zenker says, the producer commented, ''When you did the interview with Mike you did everything you tell your people to do. It works.''
Behind Zenker's approach to making it work is a law degree from the University of Pennsylvania and a successful career in radio and TV as talk-show host, news anchor man, and news manager at CBS News in New York. He has also worked as a labor relations counsel for ABC in New York. He has spent the last 10 years as a consultant in media and public communications at his own Boston firm, Arnold Zenker Associates Inc., one of about 50 such companies nationwide, according to The Directory of Personal Image Consultants, published by Editorial Services Company in New York.
In his current book, ''Mastering the Public Spotlight'' (Dodd, Mead, $15.95), Zenker pours out his ideas on communication. His approach as a consultant is unique. His goal is to help his client create the impression on the audience he or she intends.
Sitting in his posh office overlooking downtown Boston and its historic Common, Mr. Zenker could be a model for the kind of packaging his firm advocates. Trim, neatly attired in his gray herringbone three-piece suit, he projects an image that could easily command the respect of his affluent clients.
Zenker says his media background has proved to him that public appearances have to be ''produced.'' ''I keep saying that what's good enough for Art Buchwald, when he goes on the lecture circuit, should be good enough for you. . . . It doesn't mean telling jokes but . . . to say it and deliver it in a way that people will stand up and take notice.''
To penetrate what Zenker calls the ''gauze veil'' that often settles over an audience during a speech, ''the speaker must entertain them,'' he says. This means polishing his material, just as a professional does. He recommends that a speaker develop small blocks of material which can be used over again with each new audience. These blocks, polished in every detail from facial expression to gestures, can be interchanged to suit different occasions.
This is the method Zenker himself uses when he conducts workshops for corporations. A typical two-day workshop includes 10 to 15 people, each of whom pays $525 to take part. Zenker uses videotapes, role-playing situations, and exercises to loosen up his pupils.
Arnold Zenker's consulting business began in 1973, when he was playing host to an hour-long interview and variety show on a Boston TV station. An executive called and asked if he could put together a workshop to help people in his company deal with the TV reporters who were slaughtering them in tough interviews.
A somewhat surprised Zenker did it. Other executives who had gotten to the top without learning anything about the media business began to seek professional help. Zenker began by offering his services as a consultant on a part-time basis, but soon went into it full-time.
He has been called an ''imagemaker,'' a ''media priest,'' and a ''media doctor.'' Why? Because a major part of his work is helping executive officers of corporations prepare for crisis situations, such as a command appearance on ''Face the Nation.''
How does Zenker work with a client? First, he tries to acquaint himself thoroughly with the person's business. Then he formulates questions that a reporter would be likely to ask and helps the client develop brief answers. (Zenker's experience in broadcasting tells him that hardly any program will use answers longer than 18 to 30 seconds in its final cut.) Then, he and the client rehearse.
A TV interview is nothing more than an adversary situation, he assures his clients. ''There are no bad questions. But there are bad answers. They (reporters) have a right to ask whatever question they want. It's your job to be prepared to answer it. . . . Recognize that you shouldn't feel you are in a confessional.''
Practice is essential to creating the illusion for which Zenker aims. This illusion (which is not deceptive, Zenker assures this reporter) includes, first, appearing to enjoy being there with that audience, and second, appearing to know what you are talking about.
''You do what you have to do to make that experience work, not to be dishonest, not to the detriment of, . . . but for the benefit of, the audience. I don't think that anyone has the right to foist an image or an illusion of their ineptitude upon an audience. . . . I don't want to be bored when I'm in an audience. . . . I paid my money, . . . and I'm entitled to the best shot they can give me.''
Mr. Zenker's book is not a self-contained self-help book. It does, however, offer useful suggestions on content, delivery, and dress for people who give speeches. It expands on Zenker's comment to this reporter: ''Why is it that people who are in business and understand business don't recognize that public appearance is a business? Making a speech is a business - every bit as much a business as making a widget.''
During his recent ''60 Minutes'' interview, Zenker referred to himself as ''a hired gun.'' He went on to say, ''I think a hired gun can do things an insider can never do.'' Apparently acting as a hired gun gets results. Zenker pulls out an overstuffed Manila folder. ''Here, take a look at these,'' he says. ''. . . I wanted to show you how you can have an effect of people.''
One letter, from Joseph L. Koach, executive vice-president of the International Franchise Association, tells the story: ''If it were not for your services and counseling, Arnold, I'd hate to think how it (a TV interview on '' 60 Minutes'') would have come out. As it was, we did not do badly."