It's not easy being a second wife. Especially if the first one was Suzanne Pleshette.

But the bloom still seems to be on the honeymoon after three years. And what Nielsen has joined together let no programmer put asunder. So by popular demand, the happy couple is heading into a fourth year of togetherness.

And, as happens in contemporary real-life marriages, Mary Frann speaks of her TV pairing with Bob Newhart in terms of growth, change and feminine fulfillment.

"Between the first and second year of 'Newhart,' >" Frann said, "Joanna took control of the inn. I think there're more comedic possibilities that way. We've established that he's going to write books and she's going to run the inn." Rather than simply be the innkeeper's wife.

"She started out as a sophisticated New Yorker who changed her lifestyle for him," recounted Frann. "But I knew from my experience that people who keep inns look a variety of different ways. There's no reason this woman can't be sophisticated. We've started establishing that she has great taste."

Part of the woman-of-taste image allows Frann to show off her penchant for terrific sweaters. And some viewers of the show have noticed that her hair and makeup seem to have gone from plain to grade A fancy.

The whole New England inn scene that serves as the backdrop for the current allows her to feel comfortable with her real-life love of travel and first-hand experience with country inns of the world.

"I took my mother to Ireland for the first time -- she's Irish," said Frann. "I was surprised to see 'Newhart' on television there. I didn't know it was on in Ireland."

Other stops between seasons have included New York ("I like New York, always have. I did theater there"), Paris and London. But most of all the French countryside -- and of course its inns. "I loved them long before 'Newhart' came along," she said.

Los Angeles is home now, but she still visits her parents back in St. Louis, where she was born and raised.

The urge to perform struck Frann early. By age 16 she was appearing in musical revues at St. Louis' Crystal Palace, where such as Nichols and May and Phyllis Diller had preceded her.

She went to Northwestern University for two years and then, engaged to be married, she applied to and was accepted by Yale, where she would be near to her playwright fianc,e. Instead of heading off to New Haven, though, she decided to take a new job at a St. Louis radio station. She thought it would be a good way to raise money for the pending marriage, she says now. But she never got back to New Haven and never got back to the playwright. No regrets, she says.

"When I got to L.A. (to pursue her career), I was i a class," she said, the studies making up for having passed over Yale. "But you must get into a performing situation as well."

In 1968 she made her dramatic debut in the series "My Friend Tony." She also did stage work at the Mark Taper Forum and ventured back East to Broadway in 1970 to do the play "Story Theatre." And there's been more TV work -- the "Return to Peyton Place" series, "The Rockford Files, "The Mary Tyler Moore Show," "WKRP in Cincinnati," "Fantasy Island" and "Darkroom. " There were also the TV movie "Portrait of an Escort" and the series "King's Crossing."

Auditioning for the part of Bob Newhart's wife in the second series to bear his name posed a challenge since Frann had to follow in the footsteps of Pleshette, who seemed to be the definitive Newhart stage wife as Emily in the first Newhart sit- com, "The Bob Newhart Show."

"When we did the pilot," I was perfectly relaxed," said Frann. "He'd done a successful show before and had a tremendous following. And I felt the chemistry was right between us.

"It was just like a personal relationship . . . At first you meet each other and then you have to get comfortable .

"I want Joanna to continue to be a forceful person. I don't want her to be pushy or to be a person who has things totally under control."

One complication Frann has suggested to the show's producers is to have more interplay between Joanna and Stephanie, the flaky hireling played by Julia Duffy. "I'd like to see us become the next Lucy and Ethel," she said.

But whatever the twists the next season brings, said Frann, "I think she's (Joanna) expanded more and more and has become more her own person."