When actors play actors, results can be impressive

Actors are playing actors in unusual numbers on the Broadway boards this season. ''Nicholas Nickleby'' presents Vincent Crummles and Company in full and tattered array. John Barrymore rails against the player's art in ''Ned and Jack.'' And ''The Dresser'' delivers a tragicomic tribute to a now extinct species - the provincial actor-manager. The Dresser Starring Tom Courtenay, Paul Rogers. Play by Ronald Harwood. Directed by Michael Elliott.

For the constant playgoer, the next best thing to a brand new theatrical achievement is a heralded work that fulfills expectations. Such is this play at the Brooks Atkinson Theater. The performance of Tom Courtenay in the title role justifies all those laudatory advance reports from London. Furthermore, it would be difficult to imagine a finer ''Sir'' (the other principal character) than Paul Rogers, who is new to the part. Director Michael Elliott has staged a superb production.

''The Dresser'' is more than a play about a provincial star and his valet, or an inadequate troupe struggling up the mountainside of ''King Lear,'' or a bygone way of theatrical life, or the show-must-go-on tradition amid World War II air raids. The storm in this ''Lear'' is not merely Lear himself in the traditional sense. Sir can outroar the thunder and wind machines and the timpani; he can even shout defiance at overflying Nazi bombers. But he is a spent lion. Years of provincial touring, with scant reward or recognition to compensate for the attendant hardships, have taken their toll.

And so it is on the January night in 1942 when the company is preparing for yet another performance of ''King Lear'' somewhere in the English provinces. As curtain time approaches, Sir has not shown up. Unbeknownst to anyone but Norman, his dresser, Sir has had a breakdown, has stripped off most of his clothes in the street, and has allowed himself to be guided to a local hospital. When the shaken actor-manager finally arrives at the theater, it is Norman who coaxes, cajoles, and bullies Sir into his makeup and costume.

Although Mr. Harwood's plot follows a logically progressive course, the richness of ''The Dresser'' lies in its full-flavored portrait of backstage life and its deep perceptions of the relationship between Norman and Sir. Mr. Courtenay can assume what the effete Norman calls ''me best Nanny voice'' when Sir is being awkward or temperamental or just plain bullheaded. Like the playwright, Courtenay has observed the special breed of backstage factotum - gossipy, backbiting, proprietary, ready and eager to serve, to humor or to scold , to brew up a comforting hot drink, and withal fiercely loyal.

As Sir, the second-rate leader of a third-rate company, Rogers must - particularly in the Lear fragments - assume the florid excesses of an old-fashioned player. Yet the driven actor's driving force - a genuine if desperate dedication and commitment - can never be lost sight of. Rogers does not lose sight of it. In Sir's grandiloquence there is a kind of grandeur. Rogers can make Sir not only pitiful and ridiculous but also heroically moving.

Principal among those who help complete this statewise view of the provincial actor's life is Rachel Gurney as Her Ladyship, Sir's wife, a faded beauty, a survivor who resents her survivorship in a petty aristocracy. The valiant touring troupe also includes Marge Redmond as a no-nonsense stage manager, Lisabeth Bartlett as a more than susceptible extra, Douglas Seale as a quavery Fool, and Don McAllen Leslie as an out-of-place proletarian in this make-believe kingdom.

Laurie Dennett's setting for Sir's dressing room and the immediate environs gives a wonderful illusion of the musty yet glamorous world behind the stage door. ''The Dresser'' is also enhanced by Stephen Doncaster's costumes, Beverly Emmons's lighting, and Jan Gibson's sound effects. Ned and Jack Play by Sheldon Rosen. Directed by Colleen Dewhurst.

The opening of ''Ned and Jack,'' at the Little Theater, marked the impressive Broadway directorial debut of Colleen Dewhurst and the arrival of an interesting new playwright. Sheldon Rosen's work of dramatic ''faction'' has had several Canadian productions and received the 1980 Canadian Authors Association prize for the best published drama. Since its local debut last season at the Hudson Guild Theater, the play has been somewhat revised and substantially recast.

With sympathy, admiration, and a good deal of humor, ''Ned and Jack'' commemorates two remarkable American men of the theater: pioneering playwright Edward Sheldon (John Vickery) and John Barrymore (Peter Michael Goetz), star of stage, screen, and radio. Mr. Rosen imagines that Barrymore, still in black tights and jerkin, visited Sheldon, his close friend and mentor, on the triumphant first night of the actor's ''Hamlet'' in 1922.

Messrs. Vickery and Goetz draw the necessary sharp and illuminating contrasts as the two devoted friends talk and drink the night away - each facing his own long-term crisis. For the meteoric but unstable Barrymore, contemptuous of his art, it is a foreboding of ultimate mental disintegration. For the more self-possessed but momentarily shattered Sheldon, it is the prospect of advancing paralysis.

Though ''Ned and Jack'' seldom penetrates much deeper than an effective but showy emotionalism, Rosen clearly respects and values the two men and the talents they represent. Under Miss Dewhurst's sensitive staging, the performance responds to the author's intent. The cast includes Barbara Sohmers in a brief but dashing appearance as elder sister Ethel Barrymore and Sean Griffin as Ned's solicitous manservant, not to mention a magnificent white cockatoo. The cozily luxurious penthouse apartment setting is by James Leonard Joy, with costumes by David Murin, and lighting by Robby Monk.