NEW YORK -- Shouldering a $70 million Hollywood movie is one thing. Sprinting while carrying the 5-foot-8 leading lady is quite another, as Paul Giamatti discovered.

For M. Night Shyamalan's "Lady in the Water," opening Friday, Giamatti filmed underwater scenes on sets submerged in a 350,000-gallon water tank. He had to thread his way through a 20-foot tunnel, in the dark, with no breathing apparatus ... while acting.

None of that was as daunting as running, with actress Bryce Dallas Howard, in his arms. Her father, Ron Howard, had directed Giamatti to an Oscar nomination for "Cinderella Man," and now his eldest child was playing the red-haired lady in the water.

"Swimming in the tank was OK, actually. Having to do some of the running around carrying Bryce was, for me, difficult," says Giamatti, who only had to hoist wine glasses and disappointment in "Sideways." He fretted, "Oh my God, I'm going to look like such a loser if I can't do this, I'm going to slow the movie down if I can't do this." But he pulled it off.

Howard had lost some weight to make her character look fragile ("as much as my bone structure would allow"), but one day proved especially taxing.

As Howard later volunteers, Giamatti repeatedly had to carry her over water-soaked, slick plywood, up steps and through a door. "And, finally one time, he just fell and kept holding me and slid across the entire room and then we finally crashed into a wall. He was like, 'You OK?' Yeah, I'm OK," the actress recalls.

Clean-shaven Giamatti, in horn-rimmed glasses, short hair and a smart navy suit (he's not Hollywood enough to know the name on the label), is sitting at the head of a table long enough to accommodate TV's Waltons ... and a couple of stragglers. Instead, it's populated with reporters.

He may have graduated from prep school, earned a master's from Yale University, be the son of the late Yale president and baseball commissioner and be ensconced at the ritzy Waldorf-Astoria on Park Avenue, but he comes across as a regular guy. That is the secret of his charm and appeal.

Talking about speaking for a sleazy exterminator in the July 28 animated release "The Ant Bully," Giamatti jokes, "I don't necessarily think my voice is my best feature. I'm not sure what my best feature is..."

In "Lady in the Water," he plays the superintendent of an apartment building called The Cove, and he says he once worked as a janitor in a huge gymnasium, "so I was actually familiar with cleaning toilets." His character, Cleveland Heep, collects trash, changes light bulbs, pulverizes bugs and oversees the pool where a mysterious creature emerges.

There was no question he wanted to make "Lady" once he read Shyamalan's script. "He's making eccentric movies, that's what I think is most interesting, and they're commercial movies. ...

"They're very strange, and I thought that was kind of great. I thought, this is a very weird idea, and if he could pull if off, it would be amazing. I didn't know if he could. I think he does, but it's not an easy thing to pull off; it's kind of an ambitious idea."

"Lady in the Water" is a fantasy, a bedtime story come to life about a narf or sea nymph who lives in the passageways beneath the Cove's pool. Howard plays the creature, named Story.

Asked how he sees the movie, Giamatti says it's "sort of aimed around children," but his 5-year-old son is too young to see it. The boy still wants his dad, himself weaned on the "Just So Stories," to invent tales about Superman and Mickey Mouse.

"Weird kids might like it. Older kids. I would have liked it when I was a kid but I see it as more like it falls into some kind of area of fantasy, science fiction, not really horror. It's a fairy tale or a bedtime story or a myth ... but it's being told to all these normal people in an apartment building."

It's a complicated tale, with its own vocabulary and set of rules, which prompted Shyamalan to ask his actors to stick to the script. The plot points are the stars, and the audience needs to hear them, from Giamatti or other cast members, including Shyamalan, cast as a struggling writer.

"He's pretty good, actually," Giamatti says of his co-star. "He was very good at directing himself and he was actually really hard on himself, which he shouldn't have been."

Twenty-five-year-old Howard, meanwhile, is "ridiculously good," Giamatti enthuses. "She's actually an incredibly sophisticated actress, beyond my capabilities. She's so sort of expert and controlled. Really, really good ... a very sweet woman."

Her father, however, was a bit concerned that Howard runs around in a man's shirt for much of the movie, but the film is rated PG-13 for frightening sequences, nothing more. Howard, clad on this summer morning in a short-sleeved black dress with her softly layered hair back to red after playing blond Gwen Stacy in "Spider-Man 3," radiates happiness.

She married actor Seth Gabel, her boyfriend of five-plus years, on June 17 in Connecticut and is just back from her honeymoon. In addition to "Lady in the Water" and "Spider-Man 3," opening next summer, she will be seen in Kenneth Branagh's "As You Like It."

With her father, uncle and grandparents all actors and her mother, Cheryl, a writer, Howard may have been destined to go into the entertainment business. When she was young, though, she had a different profession in mind.

"I wanted to be a forensic anthropologist. But when I look back on that, I realize that had to do with the same exact reasons why I act. Forensic anthropology is about researching these different worlds and understanding how these people lived and what made them tick and what their hopes and dreams were and what ultimately made their worlds collapse."

In fact, Howard had decided she would do all sorts of research on fairy tales and present her findings to Shyamalan.

"As I started then listening to him, and not trying to be impressive, I realized, 'Oh, I just kind of got to show up and he's going to know what to tell me to do, and that's going to be the best way that we go through this process.' This is all from his crazy, brilliant, wonderful mind, and that's what I wanted to be for him, just someone who was going to allow that vision to manifest itself."

Howard can appreciate the idea that Shyamalan first spun "Lady in the Water" as a bedtime story for his young daughters. But her father would use his storytelling opportunities as pitch sessions -- good for "Willow," not good for "Ransom" ("and then the kid gets kidnapped").

"He has so many story ideas, and one of the best times of my life was 'Willow.' I remember him telling me this story, creating the story of 'Willow.' I was 5 or 6 years old and then going on that set and oh, this is amazing."

Ron Howard would prop Bryce on his shoulders while he worked, and she laughs that he's probably bald now because she would rub his head from that vantage point.

She was reminded of this warm father-daughter dynamic when she saw Shyamalan's two girls on the set, with the older one sitting next to him. "She was watching the monitor and he was watching the monitor, and I was like, 'Oh, honey, you're in for it. You got it, you got what your dad has.' "

"Lady" reunites Shyamalan with Howard, who memorably played a blind woman in his 2004 release, "The Village." Working with the director the second time was even easier.

"I'm part of a theater company in New York, and it was exactly what I'd hoped it would be because, with a company, you're able to work with people over and over again and so you get past all of the small talk and you can actually get to the essence of what we're trying to do. That's exactly what this experience was like.

"With 'The Village,' it was still like an introduction. ... Near the end of shooting, we were just oh, this is how you tick, this is how I tick. We were able to start from that place when we did 'Lady in the Water' and get deeper."

Howard improbably reaches to the Britney Spears song "I'm Not a Girl, Not Yet a Woman" to describe her character, a water nymph who is actually not a human at all. The narf has an almost childlike manner but Howard still had the dilemma of what to wear under her costume, such as it was.

"I have a tendency, just in my own personal life, to dress very modestly and to be slightly inhibited physically," says Howard, who struggled with the question of undergarments. Turned out to be a daily choice, she says.

Shooting at a former warehouse outside of Philadelphia, she found herself walking by a parade of construction workers in a wet shirt every day. They were respectful but "it was like a girl's worst nightmare."

Before she could don the shirt or blue towel, which she wears while huddled in a shower, she underwent three hours of makeup to have her freckles painted out and to make her skin very, very pale.

Howard, who pays tribute to "Spider-Man 3" co-star Kirsten Dunst -- "grotesquely talented, unbelievably talented" -- just as Giamatti complimented her, isn't angling to be a movie star.

"I don't usually want for things you can't control. I want to be the best actor that I can be. I want to be working in this business, absolutely. If that means being a movie star, OK, that's fine, but to me, movie star, celebrity, all that stuff, means something very different than being an actor."

She, however, seems destined to be both.