There are a lot of things you think about when discussing Susan Sarandon.

Grandmother is not one of them.

Yet the actress plays a grandmother in the new comedy "Tammy," opening Wednesday, and she'll become a grandmother this summer as her daughter, Eva Amurri Martino, is set to give birth to a daughter.

"I remember doing 'Anywhere But Here' with Natalie Portman as my daughter was getting older and close to going away to high school," Sarandon says. "I felt that was a primer on what not to be, but this ? if anything, I think it encourages you to think that as a grandparent you can have a special relationship that a parent can't have because the parent has to be the moral bottom line all the time and the grandparents can be a little bit outside the box. At least that's the way I'm looking at it. I hope I'm still hale and hearty when it comes time for a road trip with my grandkids."

In "Tammy," Sarandon plays Pearl, a boozing granny with medical issues that joins her granddaughter Tammy (Melissa McCarthy) for a raucous road trip.

It's an impressive transformation for the elegant Sarandon, who dons a white wig, "high-waisted, elasticized baggy jeans" and prosthetic cankles. But that look isn't something she'd ever consider keeping.

"Are you kidding me? No!" Sarandon exclaims. "All of the stuff worked very well for feeling like a grandmother. You ... stoop all over and it's just appalling what that does to your psyche."

The actress explains that she saw Pearl as being small, skinny and hunched over, wearing comfortable sandals and carrying a six pack. But when she saw the film, "I didn't look as malformed as I felt. I felt so much more extremely contorted than the way it looked on camera."

Family affair

Just as she was one of the few people to be directed by both the Scott brothers, Ridley and Tony, Sarandon is also one of the few to be directed by the husband-and-wife duo of McCarthy and Ben Falcone: Falcone acted in and directed "Tammy," while McCarthy directed Sarandon when she guest-starred on an April episode of "Mike & Molly."

"Melissa is surprising in that as present as she is when she's acting, and as brave, physical and everything else, she always has the entire film in mind as bits and pieces are flying around," Sarandon says. "So I wasn't surprised that she did a good job on 'Mike & Molly.'

"Both of them created atmospheres where you felt very safe and that were fun and none of the anxiety was passed along to you," Sarandon continues. "So you felt that you could make mistakes 100% and you could throw out ideas 100% or lines all over the place. There's very little ego with both of them. ... I'd say both of them are such nice people and such talented people and confidant and were really fun to work with."

SPiN cycle

Besides her acting career, Sarandon also moonlights as a business owner. She's a co-owner of SPiN, a ping-pong venue and bar that has locations in Flatiron (48 E. 23rd St., 212-982-8802) as well as Los Angeles, Toronto and Dubai in the United Arab Emirates.

Sarandon says that she and the other owners are working on building the franchise and giving back to the community.

"We're now, in a grown-up way, franchising," she says. "We actually hired, for the first time, a PR company, we're doing marketing and all those things.

"We actually feel, after five years, more grown-up and professional because the fact that we were successful from the very beginning meant that we never really had to learn how to do it right," Sarandon continues. "Now we have real business people who have come in that are growing the business and understand how to do it."

SPiN has also been donating ping-pong tables to underserved schools.

"We've done about 44 schools, with instructors in the area," Sarandon says. "Now 14 schools have ping-pong as a scholastic sport, so our ping-pong nation is growing. And these times when everyone is supposed to be so concerned about health and kids getting up and moving, we've found that kids really respond to ping-pong and to be able to develop that habit -- because you can play ping-pong until you die as opposed to basketball or baseball or whatever -- it has really worked."

As for the best ping-pong player in the Sarandon household, she defers to her youngest son, Miles.

"Not me," she admits. "I'm not competitive enough. If it starts to get really heated, I just give up. I just surrender."