April 02, 2014 11:00 AM

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She’s had an Office job, gotten married and is expecting her second child, but occasionally Jenna Fischer just has to take a step back to let it all soak in.

“Sometimes I can’t believe that I live in a house I pay a mortgage for — it’s all so very adult,” the actress(here in a printed Ingrid & Isabel navy maxi dress) told PEOPLE at the launch of Jennifer Love Hewitt‘s L collection for A Pea in the Pod on Tuesday in Beverly Hills.

“Now we’re going to be responsible for two people. It’s a little daunting.”

As she is “getting close” to delivering, Fischer — who is keeping baby’s sex a secret — has taken a sweet approach to preparing son Weston Lee for the upcoming addition.

“He likes to talk a lot about what big boys can do and babies can’t. Like, big boys can eat cupcakes and walk and talk, but little babies just sleep and eat and poop,” she explains.

“We’ve been reinforcing the differences between a baby and a big boy so he can be excited about eating the cupcakes — which he is.”

Fischer and her husband Lee Kirk are also showing their 2½-year-old “pictures of himself as a baby” and explaining how they cared for him when he was younger. And while Weston may be excited about becoming a big brother, he’s also very interested in the sugary celebration that will follow.

“My big pregnancy craving with him was Menchie‘s frozen yogurt and there is a Menchie’s across the street from where I delivered. After he was born, there is a picture of me eating Menchie’s [and] holding him,” she explains.

“We showed him the picture … and I said, ‘Mommy has to go to the hospital to get the baby out and you can come visit me.’ He said, ‘After the baby is out, Mommy gets to eat Menchie’s.’ I said, ‘That’s right … do you want to come to the hospital and have Menchie’s with me?’ And he said yes!”

But hopefully the frozen yogurt date doesn’t come too soon, as Fischer admits she still has a long to-do list before baby’s arrival in late spring.

“There’s nothing done. My mom and I got all the nursery supplies in order, so we have the appropriate number of onesies and diapers and a Diaper Genie — but there’s no décor,” she says. “I just ordered a chair for my nursery and I’m very concerned that the baby is going to get here before the chair arrives.”

— Anya Leon with reporting by Aili Nahas

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