For the amusement of the assemblage on the set, Tom Selleck is doing the impression again. "That's my name, don't wear it out," he says in a high-pitched squeal. Since laughter does not emanate from his current co-star, he plays the hospitable host. Ever polite, he asks, "Did you see that movie, Angela? Did you see Pee-Wee's Big Adventure?" Ever politic, Angela Lansbury replies, "No, I don't think so. Oh, wait. Was that the one in which he went back to school?"

Trouble in paradise. It's 9 o'clock on a Saturday night in a studio in the shadow of Diamond Head. Usually at this hour, Angela Lansbury is at home with her husband in Los Angeles. Instead she is now facing Tom Selleck in a bedroom. Jessica Fletcher, the mystery novelist, is sharing her suspicions with Thomas Magnum, the investigator. The script is called Novel Connection, although Lansbury jokes that Dislocation would be more appropriate. With CBS programming chief Kim Le-Masters acting as matchmaker, the two series are staging a shotgun wedding. During this Magnum, P.I.I Murder, She Wrote two-parter (airing on Nov. 19 and 23), Jessica must bail Magnum out of trouble, which is only appropriate. This crossover stunt is in fact an attempt to use Murder to raise the ratings and buff the tarnished image of Magnum. In the middle of its seventh and probably final season, Selleck's show is trying to recover from the trouncing it took for two years opposite The Cosby Show. In the middle of her series' third and most successful season yet, Lansbury is accomplishing what Lucille Ball, Ellen Burstyn and Valerie Harper have not: She is the only female TV star driving her own vehicle into the Top 10 ratings week after week. As CBS' highest-rated show, Murder, She Wrote has even eclipsed 60 Minutes.

This is a story about a medium that can make a 61-year-old English actress the hottest woman on TV—and can cool down a 41-year-old sex symbol as suddenly as he heated up. It is also about the shelf life of television celebrity, which is one of the few things that Tom Selleck and Angela Lansbury share. You can chart the difference in their respective standings as each one discusses this project. "I said, boy, we really should make this work because it will help us in the ratings, which is the only game in town," says Selleck. "And I don't know if I'm going to be doing Magnum next year." The angel of mercy had another perspective. "To be perfectly honest," says Lansbury, who isn't good at being anything else, "I was leery. I felt that taking Jessica out of her setting would tend to dilute her presence. You shred her, trying to fit her into Magnum."

Until their current co-production, Selleck and Lansbury were only award-banquet buddies. "We meet at big round tables over huge slabs of beef," says Lansbury, who first encountered her co-star at the Golden Globe Awards several years ago. Ask Selleck if he knows the movie Lansbury made in the islands 25 years ago and he shakes his head. (The answer: Blue Hawaii, in which Lansbury played Elvis Presley's mother.)

As the end-of-the-week shooting drags into overtime, the only thing not forfeited is protocol. The crew doesn't summon one star to the set until the other is ready. "Tom is on his way, you can get Angela," crackles a voice on a walkie-talkie. Throughout the week, production problems have escalated. If it's not the indoor scenery, it's the outdoor location. If it's not Lansbury's lighting, it's Selleck's schedule. Even Angela's ever-arched eyebrows have lost their quizzical expression. As you can tell from the frustration on her face, this is the first crossover for Murder, She Wrote.

"And the last," she whispers.

In retrospect Angela Lansbury thinks she should have known better. When she agreed to star in a new mystery series after Jean Stapleton declined, she was all but a video virgin. "A babe in the woods," she says in that cultivated voice that could make an obscene phone call sound like Shakespeare. "I should have done some homework." Until then Lansbury had always condescended to TV. "I figured it would burn me out, that I would become so familiar it would never enhance my career. So I turned it down—and turned it down and turned it down."

In fact Lansbury prospered outside of prime time. When she moved from the U.K. to the U.S. in 1940, she carved out a new kind of Hollywood career: youthful character actress. While her peers impersonated ingenues, she was both flourishing and frustrated playing mature women in State of the Union and piranha mothers in The Manchurian Candidate and All Fall Down. Cast as Elvis' mother, for instance, "I was ashamed because I wasn't old enough to be his mother [she was 36, he was 26]." Lansbury rehabilitated her reputation after she traded Hollywood for Broadway. When she opened in Mame in 1966, she won the first of her four Tony Awards. But Mame was later the source of setbacks, too. She lost the movie role to Lucille Ball. And after a big-budget revival of Mame bombed in 1983, she bid adieu to Broadway. "I've coasted along in my career, and then I've shot up doing something and then leveled off and gone into something else."

Going into Murder, She Wrote was a calculated career move. At the urging of husband Peter Shaw, a former agent who now choreographs Lansbury's career full-time, she circulated the word that TV was no longer anathema. "My husband particularly felt I should at least scratch the surface of TV. Financially there's a tremendous amount of money in it—and without too much work, we thought. Well, that was a big misunderstanding of the facts." After the first season, she decreed that she could no longer tolerate 15-hour days. "I just can't work more than 12 hours," she says. "I begin to fall apart at the seams." These days, in fact, she frequently describes herself as a nun to TV. But she can no longer envision a year's run on Broadway, either. "That is another form of nunnery I can do without," she says.

Jessica Fletcher has given Lansbury a chance to act herself as well as act her age. "I've never, ever played a person who was close to myself until Jessica," she says. The reason is not all virtue and virtuosity. "The proposition frightened her," says Murder executive producer Peter S. Fischer. "She was used to burying Angela Lansbury in a character. She was nervous about revealing herself. If you say to an audience, 'Hey, this is me!' and the audience doesn't like you, well, that would be crushing."

Lansbury hasn't changed her attitudes about TV acting just because of her series' success. "Series acting is a technique that is quite particular unto itself. There's nothing really required beyond being able to sustain a scene and learn your lines and have attitudes," she says. "Television acting does not require a great deal of intellectual thought or even a great deal of intelligence. It's a different style completely from what we use in the theater and mostly in the movies. It's paper thin, that's what it is. If you try to look behind the eyes of a lot of television actors, there's absolutely nothing going on."

In retrospect Tom Selleck thinks he should have known trouble was coming when he won his Emmy in 1984. "That was the year Cosby premiered," he recalls. The first question a guy asked me in the pressroom was, 'What do you think of Bill Cosby being No. 1 last week?' I'm standing there with my Emmy in my hand. I should have had an inkling."

Indeed Selleck found that nothing recedes likes success. Since Magnum had proven an instant hit when it premiered in 1980, "You get to the point where you do start to take the ratings for granted," he admits. "For a long time, I hadn't paid any attention." His series skidded from No. 6 in 1984 to No. 15 a year later to No. 48 last season. "You're reminded that success is an intangible thing," he says. "It's a good kick in the pants." It's also an instant education in the transitory nature of TV stardom. "TV can gobble up people, spit them out and go on to the next, if you're not careful," says Selleck. "You just don't know when people are going to get tired of you."

The disappointing box office of his feature films (High Road to China, Lassiter and Runaway) compounded his problems. Ever since he forfeited the lead in Raiders of the Lost Ark to Harrison Ford because of scheduling conflicts, Selleck has gone bonkers trying to go Hollywood. "Runaway [made in 1984] was a big disappointment to me," he says, "because I thought it was going to make a lot of money." Among the roles he has turned down in his misbegotten movie career is the lead in Witness, which brought Harrison Ford an Oscar nomination.

This year Selleck has concentrated on Magnum instead of movies. Last summer he chose not to make a feature, as he usually does during the show's hiatus. Instead he moved to Los Angeles and spent weeks in script conferences, budget battles and strategy sessions for the series. He lobbied the network for a new time slot and was rewarded with the mixed blessing of a spot opposite Dynasty. "Now we're just up against a hit instead of a phenomenon," he says.

Nevertheless, Selleck may still say aloha to his series at the end of the season. He is currently in the last year of his contract. He is already committed to make a movie in Australia next spring. Plans for a final episode à la M*A*S*H are in the preliminary stages. With a decision about cancellation or renewal likely later this month, Selleck oscillates between professional obligations and personal considerations. "It's just real hard putting any kind of personal life on hold anymore," says Selleck, who has maintained a three-year relationship with English actress Jillie Mack while shooting the series. "I have absolutely no social life because of the production. Everybody's had to take second place, and I don't think that's healthy. It would be nice to go out with the show being successful, but I definitely want it to go out while I still think it's good. As you can see, I'm really ambivalent."

For much of this matchup, Angela Lansbury has stewed like a lady stood up. In the backyard of a $6 million oceanfront home in Honolulu, the scheduled party scene is in suspended animation. Shaded by a tree, she sits in a director's chair attending herself with a bamboo fan. Even by the usual hurry-up-and-wait standards of TV shooting, these perpetual postponements are extreme. "Can someone please find out what they're doing?" asks Lansbury at last. As usual, her tone isn't imperial; it's professional. Selleck isn't there to answer her weary lament. He has already left the set annoyed that he was summoned before necessary. Next week, Selleck will shoot Part II with the Murder production crew. "That will be a cold shower," says Angela.

From the outset, this coupling has required a dance of diplomacy. Before filming started, there was the delicate question of where it would end. Magnum execs wanted the finale (and the ratings) for their show, although it was decided that Murder was the logical place to wrap up a mystery. A Magnum veteran wrote the Magnum outing, a Murder writer the Murder episode. Both scripts were subjected to star treatment. In the Magnum script, says Lansbury, "there were instances where I was written rather like John Hilerman [who plays Higgins, Magnum's sidekick]. They thought, oh well, Jessica is an eccentric. My point has always been that she's not eccentric." Selleck had his own complaints: "It seemed that Magnum wasn't using his wits. They were thinking, Tom can do the action stuff. But if you watch our show, that isn't always the case."

Although theirs is a cordial working relationship, it's a collision of sensibilities. A creature of the theater, Lansbury likes to run lines with her co-stars. A creation of television, Selleck likes to foster spontaneity in a scene. While Lansbury is no-nonsense on the set, Selleck is the essence of nonchalance—and that contrast of styles is evident off the set as well. While he seesaws about next season, she has already decided to sign off after a five-year run. While he worries about overexposure, she wonders about the limits of her popularity. According to executive producer Fischer, when innovative shows like Moonlighting monopolize the media, "Angela wants to know why we don't get press in proportion to our ratings. She will ask, 'Does anybody know we're out there?' "

They do today. The touring edition of The Day of the Locust that haunts every Selleck location has materialized once more. While Lansbury waits out the delay in her motor home, Selleck's assistant lines up the onlookers across the street and passes out postcards of Tom. Of her own appeal, Lansbury observes, "You can't compare me to Tom. I'm like an old shoe [to my fans]. I don't have to deal with all that."

But as she returns to the set, the bystanders notice. Although this crowd doesn't know Mame from Mamie Eisenhower, a cacophony of voices shouts out for Lansbury with a frightening familiarity. Preoccupied with production matters, Lansbury continues up the driveway without acknowledging them. "Angela, take a turn," advises her assistant, and in midstep, Lansbury spins to deliver her pleasantries while tourists' cameras click. In the throwaway world of TV, this impromptu moment could serve as an official coronation.

Says Lansbury, "I feel like Tom Selleck."