CASEY'S SHADOW Annandale, Aspen Hill, K-B, Georgetown Square, K-B Sliver, Laurel Cinema, Riverdale Plaza and Tyson's Twin.

"Casey's Shadow" is a horse movie and a Walter Matthau movie - both have their devotees, not necessarily mutually exclsively - but most of all it's a family movie. That is, it shows dramatically what it means to live in a family. This makes it also a movie "for the whole family," a term usually used to mean only that the movie has no sex in it.

The family is one which could never qualify for the psychlolgists' seal of approval. They are poor horse-trainers in Louisiana. Mama left some time ago, and when you look around the house you can see why. Daddy is occasionally foolhardy, occasionally cowardly, and perfectly capable of making the wrong moral decision. The three sons know they could do better elsewhere. Everybody is always asking how everyone else can be so dumb.

What makes them a family is that they love one another and want to work together. There are plenty of frictions in the household, but none of them are of the nobody-does-enough-for-me variety. From the seven-year-old waeily emerging from babyhood to the old putty-faced father, they are all interested in something outside of themselves - in this case, raising a race horse.

Because this basic structure is so strong, everything else seems to come naturally. The actors - Matthau, Alexis Smith, and Andrew A. Rubin, Stephan Burns and Michael Hershewe as the children - are naturalistically perfect. The characters' attitude toward horses is never oversentimentalized; they love them but expect to make good money from them. If you didn't come with an interest in the machanics of quarter-horse racing, you'll pick it up because of the characters' interest and the understatement of Martin Ritt's direction. And the portrayal of women and children - usually the falsest note when any such sporty subject is undertaken - is beautifully right.