With Casey's Shadow," an ingratiating family melodrama, and "House Calls," a diverting romantic comedy, Walter Matthau has become the most reliable box-office attraction of the spring.

Satisfying on their own terms, his performances in these new pictures could also be construed as inspirational, since they're his first outings following successful open heart surgery. He has returned in marvelous form. "House Calls" presents him in a romantic comedy lead that perfects the froll, witty charm he projected a few years back in "Pete 'n' Tillie" while accomodating several interludes of delightfully undignified farce.

Matthau can be physically funny without half trying, or seeming to try. For example, there's something ineffably funny about the sight of him placing a phone call, the receiver in one paw and a wallet in the other, opened in such a way that a chain of credit cards dangles to the floor like a broken miniature accordion. Matthau doesn't appear to exert himself to achieve most of his funny inflecfions and postures. He has simply mastered the art of getting distinctive comic music out of his own unlikely instruments, a growling, drawling voice and a tall, stooped saggy body.

Cast as a recently widowed surgeon who thinks that he wants to play the field until persuaded otherwise by a vivacious, self-respecting divorcee played by Glenda Jackson, Matthau demonstrates peerless verbal timing. He also capitalizes on physical opportunities. They range from singing in the shower to a playful contortionist act in which he and Jackson twist themselves into human pretzels, testing her assertion that screen lovers were once required to keep one foor on the floor while in bed.

Working in drag sometimes liberates unexpected comiv resources in indubitably straight actors. The classic example among modern films is "Some Like It Hot," which struck mother lodes of inventive silliness in Jack Lemmon and sophistication in Tony Curtis. Matthau gets similarly inspired when plot complications briefly force his character into public clothed in a fuzzy red robe, bandana and sneakers. The image of Matthau in drag does indeed sublimely ridiculous.

Although Matthau and Jackson maintain the romantic comedy impetus now flowing pleasantly thanks to "Annie Hall," "Smokey and the Bandit," "Citizens Band" and "The Goodbye Girl," one could not describe "House Calls" as flawless entertainment. It has slack, unassimilated passages, possibly caused by an act of script doctoring that met with only limited success.

The screenplay is credited to Max Shulman and Julius J. Epstein, who have not been all that active in Hollywood for the past several years, and Alan Mandal and Charles Shyer, a new comedy writing team. Mandel and Shyer worked on "Smokey" and the upcoming Jack Nicholson picture, "Goin' South." It appears that they were engaged to rewrite a romantic comedy that had been in inventory for a while or perhaps stitch two scripts together.

"House Calls" is bright and amusing, but it isn't seamless. The comedy material dealing with the courtship of Matthau and Jackson never quite blends with material kidding incompetence at Matthau's hopsital, where his character and a wisecracking colleague played by Richard Benjamin are struggling to prevent a senile chief of staff, Art Carney, from doing irreparable damage. From the outside there's no way of telling which comedy team originated which elements.

Matthau and Jackson are a better temperamental and stylistic match than one might have anticipated, clearly superior to Matthau and Carol Burnett in "Pete 'n' Tillie" and Jackson and George Segal in "A Touch of Class."

In at least one respect they have it all over the matches in "Annie Hall" and "The Goodbye Girl": they seem completely grown-up, free of fashionable neurotic hang-ups or would-be endearing immaturity. The nature of their romance is perhaps best expressed in the course of one exchange where Jackson, sweetly fishing for a compliment, ask if Matthau wouldn't prefer her to be younger.

"No," he protests, "I'd prefer to be younger. It's really more comfortable with an old broad like you. I don't have to keep explaining things, like who's Ronald Colman."

"House Calls" may have a special charm for couples who value the same kind of rapport.