WHITHER MODERN theater? Since the advent of film, most of the material that would have appeared on or off Broadway is much more ably shown on the screen. The greatest modern playwrights have realized this, tailoring their work for maximum theatrical impact. The less-than-greatest playwrights still have not mastered that trick, and the quite a bit less-than greatest don't even realize the obsolescence of their plays. It is into that latter category that Hugh Whitemore falls; if most new plays were like the Broadway-bound pack of Lies the question would not be "Whither modern theater?" but "Whether.

Perhaps Whitemore, who has written works for both stage and screen, has gotten the two genres confused. The premise of Pack of Lies could have created an excellent IV movie. An inspector from Scotland Yard, a Mr. Stewart (Patrick McGoohan), asks a suburban London couple if they might lend their upper floor for a bit of police surveillance work. Bob Jackson (George N. Martin) is willing to defer to the authority of Her Majesty's Representative, but his wife Barbara (Rosemary Harris) is not so sure.

When the Scotland Yard sentinels show no sign of leaving. Stewart is forced to come forth with an explanation. The object of this stakeout is Bob and Barbara's best friends, the Krogers, a Canadian couple linked to a Russian spy Stewart offers profuse assurance that he has no evidence to link the Krogers, but one must be sure, mustn't one? Barbara, despite her deep reservations about the spy business, cannot summon the courage to oppose the self-assured Stewart. She becomes sucked into the labyrinth of espionage-age paranoia; lying to her daughter, to her best friend, and even at times to herself. Barbara's life becomes invaded by suspicion the way her house is invaded by plainesclotheswomen.

Interesting stuff, but not the stuff of live drama. The conversations and confrontations are too low-key to acquire any dramatic tension, and not banal enough to reach a Pinteresque plateau of menace Chunks of the play are dedicated to give-and-takes between the various permutations of Stewart. Barbara and Bob debating the necessity, the ability, the desirability, and the morality of helping the Mr. Stewarts of this world. Every one of these dialogues rings true, but not one rings interesting.

The author traps himself between his desire to keep Pack of Lies true to life (the story is based on an actual spy case of the 1960s), and turning it into good theater. Realism wins out, but reality is rarely interesting on the stage. Bob and Barbara are conventional people, and they can do no more than have the conventional reactions to their predicament.

Whitemore's use of soliloquies in a realistic framework brings his competence as a playwright into serious question. The tone and execution of the play strive for realism, but Whitemore undercuts it with the formal monologue--one of the most contrived devices in the dramatist's handbook. Simple reminiscences intended to add dimension to the characters, these solo speeches instead subtract from the play's impact.

If the script of Pack of Lies had been made into a film, the results might have been different. The tension, paranoia, and anguish inadequately present in the text could have been drawn out and accentuated by a talented film director. Clifford Williams, the luckless captain of this production, can do no more than block the actors and leave them to themselves.

THE RIDICULOUSLY over-talented cast can only emphasize the mediocrity of the material they are performing. Martin becomes a slightly befuddled middle-aged Englishman, the most the role permits. Harris creates a magnificent portrait of a woman slowly eroding from half-hidden anguish whenever Peter and Helen Kroger (Colin Fox and Dana Ivey) drop by. Fox plays a man quiet enough and thoughtful enough that you could almost believe he has a radio transmitter in the basement (which he does). Ivy's accurate portrayal of a loudmouthed, gregarious American makes the audience cringe: it is doubly funny because she has convinced the Jackson that she and her husband are Canadian. McGoohan? What can you say about a man who can turn a casually waved cigarette into a deadly rhetorical weapon? But closet Prisoner fans should not go expecting to see No. 6 in civil service dotage; Stewart is the ultimate government spokesman, and McGoohan plays him to the hilt. The rest of the cast is good too, with Kaiulani Lee standing out as one of Stewart's employees, a quirky combination of a biker and a British mum.

All of the play unfolds within the Jackson's small suburban home, for which set designer Ralph Koltai has designed a cutaway view--with one glaring error. The top floor is only visible to the left hand side of the theater's seats. Granted, no significant action takes place up there, but if you're going to the expense to build a multi-level set, you might as well let all of the audience see it.

Theater is hard-pressed as it is to keep a loyal audience from the seductions of TV and movies. But putting quality TV scripts on the stage is not the answer.