"We shot this last little section nine years ago," actress Lyndsy Fonseca tells PEOPLE. "It was so long ago, I don't really remember what I said."
Although Fonseca has appeared throughout the series in flash-forwards as the daughter of the titular Mother, all of her scenes were shot during the CBS sitcom's first season – the only way to ensure her on-screen brother, David Henrie, looked the same age for the entire series (despite the fact that he was going through puberty at the time).
"David was growing so fast. He grew like four shoe sizes and his voice dropped, so it was very apparent," says Fonseca, 27, who filmed improvised scenes and generic reaction shots with Henrie that could be used for any occasion throughout the course of the series.
"When we started, I wasn't even old enough to drive," adds Henrie, 24. "Now, I just bought my first house. I come home … and my cousins are watching the show. It is always a funny feeling. They always say, 'Baby David is on TV!' "
While Fonseca and Henrie have been frozen in time for almost a decade on How I Met Your Mother, the two actors also have been thriving as they have matured.
Fonseca – who had been a regular on Young and the Restless before HIMYM – booked arcs on Big Love and Desperate Housewives before starring of The CW's Nikita.
Henrie, meanwhile, became most known for playing Selena Gomez's brother on the hit Disney series Wizards of Waverly Place and is about to begin filming Paul Blart: Mall Cop 2.
"I still get recognized a lot from HIMYM," says Fonseca, who recently completed the new independent dramedy Moments of Clarity. "It is kind of funny because I don't feel like it is my show, but I'm really proud of to have been a part of it."
As for the finale, Henrie plans on watching with his cousins, while Fonseca has been happy to remain in the dark about how the series will end.
"I was just in the editing room doing some [voiceover] for one little line, and I purposefully didn't want to see the whole clip of what happened," she says. "I wanted to save it to see it like everybody else."