Her 9-year-old daughter, Ava, and 5-year-old son, Deacon, don't get bored with their favourites and don't find themselves satisfied with a third viewing. They just get excited at watching it again.
So it's no surprise that the 33-year-old Oscar winner is particularly concerned with the quality of children's entertainment, choosing to work back-to-back on major animated feature films.
"Obviously, having two young children it's nice to be able to go to the movie theatre and have a film there that's OK for kids, but it also has some adult humour in it that kids won't necessarily catch," she says from her Los Angeles home. "It's great to do those movies, because I have to watch a lot of them."
The first of those movies to be released is Monsters vs Aliens. In it, Witherspoon lends her voice to would-be bride Susan Murphy, who is struck by an asteroid with radioactive material that turns her into a 15m-tall woman renamed "Ginormica" by her new government captors.
The film is intended to evoke early sci-fi flicks of the 1950s and '60s.
Several years ago, Witherspoon met DreamWorks Chief Executive Jeffrey Katzenberg and his staff pitched Monsters vs Aliens with one roughly sketched image: that of a giant woman sitting on the roof of a petrol station.
"I thought it was really cool because I had always liked those movies," she recalls. "My dad and I always used to sit up late watching them."
Witherspoon signed on to that movie and soon agreed to a second animated project, The Bear and the Bow, a Pixar film set for a 2011 release, in which she gives voice to a mythical Scottish princess.
The actress, last seen in the romantic comedy Four Christmases, says she was attracted not only to the content of the films, but to the endeavour itself.
"It's a very different process," Witherspoon recalls. "I got into a sound booth probably once every couple of months for three or four hours."
And though it takes less time, she adds, it requires much more imagination. "There is nothing visually in front of you to inform the scene. And you don't have a full script in front of you, so you don't even know what's going to happen next.
"They kind of talk you through everything, and then you just go in there and play around with it."
In Monsters vs Aliens, Witherspoon was cast with some of Hollywood's funniest actors, who play fellow monsters, their rival aliens or, in the case of Stephen Colbert, a vapid US president. The creatures are voiced by Seth Rogen, Hugh Laurie, Rainn Wilson and Will Arnett, though Witherspoon never sat in the same recording studio with any of them.
"It's really difficult; you don't have a lot of actors around you to bounce off of," says the film's co- director, Conrad Vernon.
Witherspoon "was great at ad-libbing and coming up with different things, but when you're in that booth and you're doing something funny and you don't hear anything but silence, it can be daunting".
Vernon says Witherspoon was right for the part because she's "not only a phenomenal actress, but she is also a very petite woman, and has this very, very adorable voice. And to put that voice into a 49-foot 11-inch-tall giant was a great contrast and fun to do."
Witherspoon did have a couple of hours in the sound studio with Paul Rudd, who plays her narcissistic fiance. The two will be reunited for her next project, a comedy directed by James L. Brooks that adds Owen Wilson to the mix.
"I couldn't be happier," Witherspoon says. "I'm very, very lucky, and I get to work with some really great people."
She got to see Monsters vs Aliens recently in a cinema full of kids, her own two among them. "They loved it," she reports. "They thought it was so awesome."
So in that regard, at least, it's a Mum-mission accomplished. - Ellen McCarthy
- Monsters vs Aliens opened in cinemas last week.
