Paula, the title character of "The Goodbye Girl," opening today at five area theaters, is an ex-chorine of 33 who returns to her West Side apartment one afternoon with her 10-year-old daughter Lucy to find a Dear John letter on the mantelpiece. She has been left high and dry by her lover an actor who casually informs her that he has skipped to take a role in a new Bertolucci movie being shot in Italy. Soon Paula, portrayed by Marsha Mason, gets another shock: The cad has sublet the apartment to an actor friend from Chicago.

The new tennant, Elliot, arrives late at night in the middle of a downpour. He is not much to look at - scruffy, rumpled, slightly flabby and completed soaked - but in the irresistibly lively person of Richard Dreyfuss this unwanted roommate is destined to brighten up the apartment, the lives of the heroine and her daughter, the movie and the current holiday season.

In fact, Dreyfuss makes Elliot such a wanning image of a dedicated, Jenny, live-write actor that it would be preferable if Neil Simon made Guicker work of the Plot-stoking, char- [WORD ILLEGIBLE]introducing scenes in the early going.

One recognizes the playwriting [WORD ILLEGIBLE]behind the scenes in which Paula tries to put off Elliot, they [WORD ILLEGIBLE]they come to a truce, and she persists in acting suspicious or prigish after he moves in. But you want Simon to clear the deck faster sof that its exceptional leading actor can have more room to operate.

"The Goodbye Girl" evolves into the most satisfying comedy Simon has written directly for the movies. One lerates the plot mechanics for the [WORD ILLEGIBLE]of the genuinely amusing aspects of his script, the bright remarks and the distinctive or appealing charater [WORD ILLEGIBLE]that provide good performers truth live ammuniton.

Dreyfuss is certainly the most active and expert marksman in the cast. This tempo is so heightened and his delivery so crisply humorous that the [WORD ILLEGIBLE]can't seem to come fast ecough for him. He's capable of scoring vertical bull's-eyes quicker than Simon can load fro him, but there are several massages in which their teamwork, no doubt enhanced by director Herbert Hosss debonair sense to timing, seems pretty dazzling, especially during arrange of self-deprecating nifties the hero tosses off the morning after his lay, a fecklessly "experimental" procaution of "Richard III" has bombed.

If the role of Elliot doesn't lead to a long overdue first Oscar nomination for Dreyfuss, I don't know what would. Simon gives him a flirtatious line which Elliot asks Puala, "What do you think I've got? I don't mean talent-wise. I'm very secure talent-wise. Just appeal-wise I'm a little shaky," Dreyfuss' performances in "American Graffiti," "The Apprenticesphip of Duddy Kravitz" and "Jaws" left no doubts about his acting ability, although "Inserts" raised a few doubts about his judgement.

Dreyfuss makes Elliot romantically appealing in a way that is fairly common in real life but rarely acknowledged in movie romances. The source of his sexual superiority is not goood looks and an impressive physique but high spirits and a quick wit. Dreyfuss probably created a superfluous handicap by coming into the role a bit overweight. Elliot is supposed to be, but his rumpled quality would be a sufficient fake-out.

"The Goodbye Girl" is conceived at the astutely modest level, of most of the recent romantic hits. Like "Rocky" and "The Late Show" and "Annie Hall," it's a love story about oddballs and ugly ducklings rather than shimmering divinities.

The filmmakers seem to sense the same distrust in the audience that modulate what used to be soaring notes of romantic affirmation. The affirmation is still there - the whole point of the story is that unlucky aula has gotten lucky the third time around - but it's expressed in a sub-dued, slightly skeptical tone.

Simon makes things a little difficult for Marsha Mason, his own wife, by insisting the Paula resist Elliot much longer than the audience will be able to. One can't quite go along with the pretext that her previous romantic setbacks justify suspicion about such a delightfully eligible and entertaining suitor. The effect is closer to the sort of puzzlement created in "Pete 'n' Tillie" by the Carol Burnett charater's lack of response to obvious wittiness of the Walter Matthau character. One begins to suspect that these heroines are not so much cartious as backward.

Mason is very nice in sustained intimate moments with Dreyfuss and in an audition scene where she encounters an old director acquaintance who tries to console her for failing to make the cut. Her promise to "work at it" when he tells her that he's casting mostly younger dancers seems a funny and touching moment of characterization.

As one might have anticipated, Ross depicts this theatrical setting with a condident ironic touch. As far as it goes. he's also pretty good with the camped-up version of "Richard III" that spiols Elliot's New York debut. As a matter of fact, I'd like to see more of this fiasco, evidently suggested by a Joseph Papp production of a few years ago in which Marsha Mason was cast as Lady Anne. Excerpts from apocryphal disaster-prone shows are usually great fun on the stage and screen, but there's barely enough of this misguided "Richard III" to justify relishing the gag. Watching Dreyfuss, you get a sneaking suspicion that Elliot might have been entertaining enough to save the show.

Ross gives Somon's material an admirable sense of pace and balance on the screen. The amdulations between crackling comic encounters and sub-dued, intimate interludes are very graceful and attractive. Ross seems to have an experienced runner's instinct for when to sprint and when to relax.

Unfortunately, some episodes remain irreparable stumbling blocks. Although Simon is once more using New York as a picturesque setting for romantic comedy, he hasn't gotten the jaundiced, carping notes from "The Out-of- Towners" and "The Prisoner of Second Avenue" completely out of his system. They reappear in an unwisely prolonged sequence about a mugging and a gratuitous episode at a topless bar, a locale one doubts the author has been frequenting, even in the interests of research.

The welcome return to a New York for lovers is symbolized by the fadeout, in which a happy marsh Mason clutches Richard Dreyfuss' guitar to her bosom in the pouring rain. This sight is not as rapturous as its obvious inspiration - Audrey Hepburn clutching her no-name cat in a studio rainstorm at the end of "Breadfast at Tiffany's - but it's an endearing reminder. "The Goodbye Girl" itself represents a satisfying step back in the right direction for the purposes of light, optimistic film romance. Its appeal isn't exactly novel, but it is ingeniously and refreshingly traditional.