"I fake it like any working mom, and I run out the door," Garner told PEOPLE at the premiere of her latest film, Ghosts of Girlfriends Past, at the Grauman's Chinese Theatre in Hollywood on Monday night. "If you looked in my bag, there's a sippy cup in there right now!"
Garner, 37, and husband Ben Affleck became parents for the second time in January with the birth of Seraphina Rose Elizabeth Affleck. Big sister Violet is now three.
Being a mom to a toddler and a newborn presents new challenges for Garner, especially when she's on the set.
"[I] feel like half my brain is somewhere else all the time, but when the camera's rolling, I pull it together and focus for two minutes," she says, "and then I kind of turn back to a ditz again. I have a split personality."
Despite the challenges of motherhood, Garner couldn't be more proud of her girls. Whipping out her cell phone on the red carpet, she showed pictures of her kids to friends. "She's so smiley!" Garner gushed as she did an imitation of Seraphina.
And she never let her coworkers see her sweat. "She's such a good mom. I admire her so much," costar Lacey Chabert told PEOPLE. "Her little girl is so smart and you can tell she really spends time with her. I just admire her that she can balance all of it."