ON A COLD RAINY SUNDAY MORNING AT A SOUTH SIDE HOSPITAL, the actor Tim Robbins is in the midst of making a movie. But the immediate business appears to be knickknacking, paddywhacking and giving the dog a bone.

Surrounded by children, chalkboards, stuffed toys, doll houses and a gerbil, the 33-year-old Mr. Robbins, who is best known for his role in the 1988 baseball film "Bull Durham," is dressed in a dark blue pin-stripe suit, in character as his creation, Bob Roberts.

"Bob Roberts" is a mock documentary about a right-wing businessman-cum-folk-singer from Pennsylvania who becomes a popular hero and runs for the United States Senate. It is also Mr. Robbins's debut as a screenwriter and film director. This particular setting, a hospital playroom, provides the title character with a classic campaign photo opportunity as he plays his guitar and leads the children, some of whom are in wheelchairs, in songs like "This Old Man" and "Skip to My Lou."

Whether by coincidence or by design, Mr. Robbins has found himself surrounded by family this weekend. When he steps out of his role as actor and prepares to direct other portions of the scene, his brother and musical collaborator, Dave Robbins, plays guitar and continues the singalong, to keep the children's energy level high for the crowd shots.

Among the young actors on the set is 6-year-old Eva, the daughter of the actress Susan Sarandon, with whom Mr. Robbins has a 2-1/2-year-old son and a second child on the way. When the singalong scene ends, Eva is held first by Mr. Robbins and then by her uncle, Terry Tomalin, who is Ms. Sarandon's brother, a newspaper reporter from Florida. Before the day's work ends, Ms. Sarandon, who is in town acting in another movie, "Lorenzo's Oil," arrives on the set with the couple's son, Jack, who sleeps through the whole thing.

MR. ROBBINS SEEMS TO ENJOY this family togetherness, but he won't talk about it. "I never signed anything that said I give up my privacy when I become famous," he points out in a soft, well-mannered voice. "Some people think that it's your obligation to talk to people you just met as if they were friends. I get embarrassed when I read stuff like that."

He would prefer to talk about politics, film making and theater, the passions that consume his public life.

"I think people have had enough," Mr. Robbins says, referring to the past decade of conservative politics in this country. "Once they start seeing past the sound bite to what these people" - the conservative politicians - "really feel, then they'll start voting with their minds again."

Ms. Sarandon, for her part, has lent her name to various political causes, including the proposed equal rights adendment, nuclear disarmament and abortion rights. Although in 1983 she told a reporter, "I wouldn't consider myself political," by 1991 she had reconsidered her position. "If I'm gong to be treated as a commodity," she said, when asked about celebrity activism, "I might as well take advantage of it."

Mr. Robbins began filming "Bob Roberts" in November, the day before Pennsylvania's interim Democratic senator, Harris Wofford, won an unexpected victory over Richard Thornburgh, the former United States Attorney General and former Pennsylvania governor. Those results buoyed Mr. Robbins and his company, supporting their convictions that their satire may be timely and on target, reflecting a growing discontent with right-wing politics.

Mr. Robbins wrote the first draft of the screenplay for "Bob Roberts" five years ago, directed a short version of it as a sketch for "Saturday Night Live," and spent the next few years trying to get it produced as a feature film.

He admits that the script was not ready to be filmed until about two years ago. He contends, however, that at least some of the difficult finding a backer had to do with its political content.

A COMBINATION OF THE FILM'S MESSAGE and Mr. Robbins's clout as a recognizable actor convinced Working Title Films, a small independent British company, to finance the project. Still, the film's budget is less than $4 million, and the actors are working for scale, not only the co-stars, Alan Rickman and Giancarlo Esposito, who play a political handler and a journalist, respectively, but also some well-known individuals who are doing cameos, including James Spader, John Cusack and Gore Vidal (as a United States senator).

Because of his outspoken liberal views, which permeate "Bob Roberts," Mr. Robbins worries that it will be seen only by those who already agree with its politics. An even bigger fear is that its many original songs, which he wrote (with his brother) and performs on screen with a convincing folksy twang, will be taken up as right-wing anthems. In an effort to forestall that, he has retained sole decision-making power over whether to produce a "Bob Roberts" soundtrack. And if he does, he says he will change the politics of the lyrics on the album "so they're not offensive or mean."

Mr. Robbins learned about traditional folk music from his father, Gil Robbins, a musician who moved his family from California to Greenwich Village in Manhattan when Tim, the youngest of four children, was 3 years old. The elder Robbins performed for a time with the Highwaymen (whose hits include "Michael, Row the Boat Ashore" and "Cotton Fields") and later managed Gaslight, a celebrated Village folk club.

"I remember watching him on-stage and being very impressed by it," Mr. Robbins says. "I was always around pretty interesting people growing up."

By age 12, Mr. Robbins had become invloved in an Off Off Broadway theater group, doing summer street theater, political vaudeville and what he called "strange plays." He studied theater at the State Univesity of New York at Plattsburgh, then transferred to the University of California at Los Angeles.

There, he and some colleagues grew disenchanted with the school's conservative theater department and founded the Actors' Gang, which performed - and still performs - avant-garde and original works. Some are co-written by Mr. Robbins, who continues to serve as the company's artistic director. His plays include "Carnage, a Comedy" (which was performed at the Public Theater in New York in 1989), "Alagazam, After the Dog Wars" and "Violence: The Misadventures of Spike Spangle, Farmer."

Mr. Robbins has not abandoned the movie mainstream, however. He recently finished shooting "The Player," a satirical film about Hollywood directed by Robert Altman that is scheduled for release this spring.

WHETHER HE'S WORKING on a big-budget film or an experimental play, Mr. Robbins doesn't consider the political message his only priority. "Entertainment is the key," he emphasizes. "People want to enjoy a good laugh and enjoy being stimulated in some way. If they agree with you, fine. If not, fine, too.

"I'm not trying to tell anyone in my plays or in 'Bob Roberts' what the answer is," he adds. "What I'm trying to do is raise questions. Why is Bob Roberts popular? Why is he going to succeed?"

And how would Mr. Robbins answer his own questions?