The Friends with Kids actor-director revealed that Wiig filmed her scenes while still working on Saturday Night Live and finishing her own film Bridesmaids.
"I think we got Kristen on all of her days off... And she had to go to LA a few times to look at cuts for Bridesmaids," Westfeldt told BlackBook.
Westfeldt added that Wiig wasn't the only Friends with Kids star who had to work around their busy schedule to shoot the romantic comedy.
"I remember looking at the schedule and thinking, Well, Chris [O'Dowd] needs to be back here, and Maya [Rudolph]'s pregnant," she explained. "It was tough to get the people we wanted all together at one time."
Westfeldt previously claimed that many people told her budgetary issues and the sporadic shooting schedule would make it impossible to ever produce Friends with Kids.
Friends with Kids is now playing in the US and opens on June 29 in the UK.
> Jennifer Westfeldt: 'I thought I would have had children by now'
> Jon Hamm: 'Jennifer Westfeldt is adorable and frustrating'
Watch a trailer for Friends with Kids below: