IT was the day before Christmas, and for the first time in quite a while Harry Morgan was barely stirring. His latest TV series, "Blacke's Magic," was taking a holiday from its hectic shooting schedule, and there was time to reflect on what the new show has to offer, as well as on the fruits of a career largely spent playing second banana to some of the best in television's bunch.

The current top guy in his life is Hal Linden, who plays magician Alex Blacke. Morgan is Alex's father, Leonard.

"They tell me there's good chemistry between us," said Morgan, "and that's important -- witness 'M*A*S*H.' They even had a chemist from UCLA come over to test the show."

Should a chemist actually come to the set and analyze Morgan, it would be obvious what kind of chemical agent Morgan is: He's a catalyst. He is an actor who makes the top-billed star look good, steals a few harmless scenes for himself and facilitates the reactions and interactions of a show's other players.

He has played that role in more than 100 films ("High Noon," "What Price Glory?" "The Glenn Miller Story," "The Shootist," "Support Your Local Sheriff") and in nine television series.

In his 10th series, he plays a man quick with a wisecrack, a veteran of the carny circuit who will leave you wondering for a time just what kind of man he is. Turning down an invitation to join his son at a dinner, he quips, "I've got pigeons to pluck Saturday night."

"He's a low-grade flim-flam man," said Morgan of Leonard Blacke. "He's been in jail. He's a con man. Sort of a rascal."

Linden is the show's main figure, a retired illusionist enlisted by the authorities to help solve crimes in which how it was done is as interesting a mystery as by whom. Linden carries the show, with Morgan offering his customary assist. The series opens with a two-hour pilot episode Sunday at 9 on NBC and then moves to its weekly timeslot at 9 Wednesday.

"I don't have as much riding on it as he does," said Morgan. "My approach is more casual. I wouldn't call it indifference, because I try as hard as I can."

Morgan has worked with several dominant series leads and recalls the associations with relish. Richard Boone ("The Richard Boone Show," "Hec Ramsey"), Jack Webb ("Dragnet") and Alan Alda ("M*A*S*H") were all in their ways strong-willed TV men with a firm idea of how to do things. "They weren't fearful of competition, and they handed you some of the juiciest things in the show," said Morgan. Linden is showing signs of being the same kind of guy.

In one "Blacke's Magic" segment, he and Linden set off to find a place important to Leonard Blacke and to Alex's mother. "We go back to find some fairgrounds, to find the spot where I proposed," said Morgan. "It gets kind of emotional. It's a hell of a scene. And it's all mine. Hal didn't object a bit."

In creative terms, the show is a cousin to the highly successful "Murder, She Wrote." Its executive producer is Peter S. Fischer, who holds the same job with "Murder." The series was created by Fischer, Richard Levinson and William Link, who are responsible for Angela Lansbury's hit. This trio seems to know something about assembling a murder mystery series with a middle-aged-and- beyond cast. Like "Murder, She Wrote," "Blacke's Magic" is a solve-along, with viewers knowing no more than the Blackes do about who did it and how.

To keep a mystery plot moving along, at the same time being somewhat funny and light-hearted, puts a strain on TV writers, a burden that was apparent in the first season of "Murder, She Wrote." "The endings," where all the pieces fall into place, "are hard to make consistent," said Morgan. "It's true on 'Murder, She Wrote,' and it's true on our show too."

On the night before Christmas, Harry Morgan turned his thoughts to Washington, the jump-off point for his acting career. The man who was born Harry Bratsburg 70 years ago in Detroit had given little thought to acting until he came here to sell office furniture. Yes, he'd won the state high school oratory championship back in Michigan. But acting?

In the '30s, though, it was hard to sell the Shaw-Walker line of equipment. "The only ones in town who were moving office equipment in the teeth of the Depression," he recalled, "were the people selling filing cabinets to the Social Security Administration."

So he tried out at the Civic Theater, "just because I was lonesome," and won roles in "The Petrified Forest" and "Front Page." The plays were produced on a stage at the the Wardman Hotel, now the Sheraton Park.

That exposure led to a summer stock run in Mount Kisco, N.Y., where he made his professional debut in "At Mrs. Beam's." Frances Farmer and Mildred Natwick were in the cast.

By 1942 he had moved to California, where he caught the eye of a talent scout. He signed with Twentieth-Century Fox. One of his earliest movies was "The Ox Bow Incident" with Henry Fonda. When television became an everyday reality in the '50s, Morgan was ready. For five seasons, from 1954 to 1959, he played Pete, the wise-guy next-door neighbor to Spring Byington in "December Bride." His unseen wife, Gladys, was often the target of his barbs, but for two seasons (1960-62) she became visible in the form of Cara Williams in the "Pete and Gladys" spinoff.

Then there was the short-lived "Kentucky Jones," in which he was an assistant to a veterinarian played by Dennis Weaver; from 1963-64 he was part of the repertory company of "The Richard Boone Show," a one-season anthology series. "I was particularly fond of Dick Boone," said Morgan. "I started to direct with him." That experience led him to direct about one "M*A*S*H" episode a season.

For two seasons he was Jack Webb's sidekick in one of "Dragnet's" incarnations; and he was a chief deputy district attorney in "The D.A.," like "Dragnet," a Webb creation. "Hec Ramsey" followed, and there was the failed "After M*A*S*H." "We did 40 episodes of 'After M*A*S*H,'>" said Morgan. "None of them seemed to work. I think the comparison with 'M*A*S*H' hurt it. That's probably the real reason it didn't work, though it wasn't that good a show on its own."

As Col. Sherman T. Potter for the final eight seasons of "M*A*S*H" Morgan was the definitive catalyst, adding texture to one of the finest ensemble casts ever assembled.

In a business in which friendships often end at a series wrap party, Morgan is pleased to have an important carry-over from the "M*A*S*H" days. "Loretta Swit called me from London," Morgan said on the day before Christmas. "I think she's probably my best friend. She didn't even call collect."

After that call, Morgan was heading over to visit another friend, Ralph Bellamy, who had just signed to play Franklin D. Roosevelt, again, in the maxi-miniseries "War and Remembrance." "They start shooting next month, I think," said Morgan, "and Ralph's in all but one segment. But he doesn't start work until January of '87!"

And later on the day before Christmas, Morgan would go to the home of his oldest son, Chris, producer of "Dynasty II: The Colbys." His other three sons would be there too -- Charlie, a lawyer with MCA; Paul, also an attorney, and Danny, just out of law school. Eileen, Morgan's wife of 45 years, died in February.

Christmas Eve would be quite a family gathering, including the four sons and eight grandchildren. "But of course," he said, "there'll be a figure missing."