I haven't watched much How I Met Your Mother, but from what I gather, it's the story of sexual escapades ending in pregnancy one man very candidly tells his teenage children over the span of nine TV years. Given that time is capable of drastically altering people's physical appearances, the creators of HIMYM planned ahead for the reveal of the show's main conceit way back in 2006 and filmed the climactic scene with the cast that will air on the hour-long series finale episode on March 31st. From HitFix:
Series co-creators Carter Bays and Craig Thomas knew from the beginning exactly how that question would be answered, and always planned it for the show's final episode. And the kids on the couch would be a key component of that ending.
Yet in 2006, with the series in its second season and the possibility existing that it could be on for several more years, Bays and Thomas realized they'd better film that final scene right away. After all, the actors were growing up, would soon look different and wouldn't even be able to fit in the clothes they wear while sitting on the couch.
So one day Bays and Thomas brought the young actors in, closed the set to all but one camera operator, and filmed the two-minute scene that will bring the series to a close. Henrie and Fonseca signed non-disclosure agreements. So intent on keeping the secret, Fonseca put it out of her mind and doesn't even remember what they filmed, Thomas said on Wednesday.
Through all the seasons that have come since, there hasn't been any reason to change it, he said.
"It's been the plan all along," he said. "What you see on March 31 has been the plan. We leave the series with a certain message that we wanted to convey."
Since filming their HIMYM scenes, teen actors Lyndsy Fonseca and David Henrie have grown into slightly older twenty-somethings. Fonseca has appeared on Big Love, Desperate Housewives, and both Kick-Ass movies, and Henrie starred in the Disney Channel show Wizards of Waverly Place and appeared on Arrested Development and Grown Ups 2.