Melissa McCarthy and Susan Sarandon set off on a road trip for Tammy, and their cross-country antics are just a little bit sillier than Thelma & Louise.

When McCarthy’s actor/screenwriter husband Ben Falcone concocted an offbeat idea that would send one of the actresses’ comic creations – this time, a minimum-wage everywoman whose false bravado is shaken by the twin losses of her fast-food job and her marriage -- on a calamitous car trip with her unconventional grandmother, the couple decided to shoot for the casting moon: Not only did they snare an all-star lineup for Falcone’s directorial debut that includes Kathy Bates, Mark Duplass, Gary Cole, Alison Janney, Sandra Oh, Nat Faxon, Toni Collette, Sarah Baker and Dan Aykroyd, they persuaded Sarandon to ride shotgun as Grandma Pearl.

McCarthy, Sarandon and Falcone recently joined a group of journalists to break down how their kooky car trip – which included stints on a runaway Jet Ski and turns as brown bag-disguised bandits – put the comedic pedal to the metal.

On Tammy’s unlikely origins as a literal dream girl:

Melissa McCarthy: Ben came downstairs, just having woken up, and literally said, “I had a weird dream, and I think I have to write it. You go on a road trip with your grandmother and she drinks and she sleeps around. So I'm going to go write that movie.” And I thought, “All right, why don't you do that?” That was about six years ago, and that began the whole thing. He says things, and I say, “That sounds great!” I just agree with him, and it all works out. [Laughs]

Ben Falcone: The whole thing was a delight for me. It was great to work with Melissa. We'd worked together before, but we got to spend a lot of time together. And then when you add in all these great people at this table here, it was so fun to work with everybody. So literally, I can honestly say that at the end of every day, I was honestly disappointed that the day was over and hopeful that they'd let us keep shooting tomorrow.

McCarthy: One of the things I loved about the character Tammy is that kind of confidence, right or wrong. In her world, she believes it, and I always love playing that. I don't think someone has to be justified because her point of view is she was great. And the fun of playing that character is that even she, throughout the film, realizes that I'm perhaps doing a lot of things poorly.

On recruiting an outspoken Oscar-winning actress to play to play an outspoken booze-swilling grandma:

Susan Sarandon: Ben and Melissa called me and prepared me for the script, and I thought it was really fun. I was doing another movie, and I didn't quite understand what reality we would be in, so we had another call. I was just so excited by the way they worked … I thought, “I'll jump.” I was a little worried because there wasn't much time between the other film I was doing; I did three in a row – all alcoholic, pill-popping characters, so I was prepared in that sense. And we texted little pictures of possible looks and figured out the age thing, and actually she wasn't much older than I actually am – she just doesn't have my makeup and hair people! And I just thought, “You know what? They're both so talented, and it would be different – I don't quite know what's going to happen, but let's go for it.”

McCarthy: One of the first questions when we were first meeting on the phone, Susan said, “Are you seeing like a little, old granny with glasses and a crocheted sweater and updo bun?” And we're like, “Oh, God, no! She has raging problems with alcohol, and she sleeps around.” And then it was like, “OK, we'll be fine; we're in a real realm here.”

Falcone: We were just trying to figure out the best characters we could write that we thought would be the funniest, and the most affecting in some way … I never thought like, “Oh, let's make a subversive movie.” I just was trying to think, “Let's do a funny movie.” And Melissa just picks a character. In any of the movies that so far have been commercially successful, I'm pretty sure she never was like, “Ooh, I've got a really commercially successful idea for a character.” I think she's just like, “Oh, I think I want to look like this and be like this, because that means something to me.”

McCarthy: We worked on it a long time before we actually got the chance to show it to people and make it. And by the time we were ready to show people, we'd had it for years and we knew these people. I felt protected with them. So I think if somebody wanted like a bigger scene or a bigger trailer moment, we just knew that that person maybe wouldn't do that. It doesn't have to be bigger or flashier – it just has to stay in the right realm of the story. And for me, if you can kind of walk that line, more eccentric characters can push pretty far, but if you stay on the side of reality, it's always funnier. … So we tried to let all these people push as far as they could but keep it real, and hopefully, the story has more impact that way.

Sarandon: You rarely get in a situation where there's such depth of field in terms of all the supporting actors and everybody, where you feel safe enough to do things. And you want to please Ben so badly, but even when it's not working, he never makes you feel like you failed, so you can make your mistakes a hundred percent – and that's really, really liberating and fun to be able to just go with it and suggest stupid things. And one out of 10 maybe is a good idea. And a line and there were constant alternates going on all over the place. For me, it was just a really wonderful experiment in a way that I hadn't worked on film, certainly in a long time. If this movie's not a success, I'll never do it again! [Laughs] But if it was as much fun for everyone else as it was for us doing this, then it works.

On whether any real-life grandmothers’ reputations were harmed in the making of this film:

Falcone: The parts of my grandma that I used were the parts where she's really smart, really fastidious and precise, which was all stuff that Susan was able to play so well, and then she brought the boozy, wild part to it.

Sarandon: I brought my rug. My grandma had a rug, so we added that. That was an homage to my grandma.

McCarthy: Nothing in Susan's character in terms of the drinking or the men stuff [came from my grandmother], but I loved that no matter what, even if they weren't meshing up at that moment, the bottom line was she loved Tammy. I feel like no matter what – I was not at odds with my grandmother, but you knew, even if I got in trouble with her, she may scold me when I was little, but I knew she loved me. So that was a big part of it to me – there was nothing you could do to make the love go away, even if they were at odds.

On whether one has to actually be incredibly coordinated to pull off awkward pratfalls:

McCarthy: No. I think I pretty much prove that you do not have to be. You just have to be willing to basically really bruise yourself, and I have bruises. Just I'm covered, really. The worst fall I took during Tammy, we weren't even shooting it. But I did a test run on the Jet Ski – and I have only been on one, one other time, and that was 10 years ago on our honeymoon – and I was going about 40 miles an hour, which now I realize maybe not the best idea for someone who really doesn't know how to drive one. And the lovely man that was teaching me said, “At the end on your big, wide turn, once you slow down, turn into it.” I just heard, “Turn into it,” so I cranked it up to 40, and I thought I was supposed to do these S-curves, and I flipped myself off it so hard. I lost our first wig – we had a very tiny budget, and they're like $7,000. So I didn't know I was off the Jet Ski, I didn't know I had fallen – it was that fast. It just flipped the whole thing and I was underwater, and the first thing I went for is the wig. We were doing camera tests, so I was in full costume and I came up, once I figured out which way was up, and I said, “At least, we got it!” And they said, “We didn't turn the camera on yet!” I was like, “Nooooo!”

On the intricacies of acting beneath the brown paper bag disguises:

Sarandon: That bag – I don't know if it's as funny when you're outside the brown bag, but inside the brown bag, looking at Melissa and putting your head together and whispering, “Just take the money? Do we take the bag?” We couldn't get through a take. I don't know why that seemed so funny.

McCarthy: I just watched the one where we couldn't get through it. It makes me laugh so hard, like, I feel slightly crazy.

Sarandon: I would suggest seeing the movie with a bag over your head, because there's something about that bag that just broke me.

Tammy opens today nationwide.