The "Mamma Mia!" star was spotted filming scenes with both a real baby and a prop doll, reports dailymail.co.uk.
In between takes, she was seen talking to first time director Amy Koppelman and even kissing a puppy as her colleagues laughed.
The "Mean Girls" actress flaunted her taut midriff in a white tee worn under a checked shirt and a denim jacket. She also wore a pair of washed out baggy jeans with rolled up bottoms and a pair of wedge heels.
"A Mouthful Of Air" is adapted from the novel of the same name by Koppelman, who wrote the adapted screenplay and is also directing.