'This Is Us': Who Is the Mystery Man at Rebecca's Door? Mandy Moore and Producers Tell All (Exclusive)

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Warning: Spoiler alert! If you have not watched Tuesday's season three premiere of This Is Us, do not proceed.

While we wait for answers on one mystery, This Is Us introduced another unanswered question to speculate over.

The season three premiere went back to the night Jack met Rebecca for the first time in December 1972 and followed the pair as they went on their adorable, but awkward, first date at the local carnival. Though we know they eventually end up falling in love, marrying and having kids years down the line, their initial courtship wasn't as smooth as we might have assumed. So much so that when Jack drives by Rebecca's house the following day to return her jacket, he's just a few minutes too late.

Why? Because there's another guy -- mustached, blonde and with flowers -- at Rebecca's doorstep and he's kissing her.

So who is this mystery man at her door? His name is Alan and he's played by Weeds and Good Girls Revolt star Hunter Parrish.

Hunter Parrish plays the mystery man at Rebecca's doorstep at the end of the season three premiere of 'This Is Us.'

NBC

When ET spoke with Mandy Moore about the moment, she admitted she was shocked at the arrival of the new character. "I was somewhat surprised, but that's [creator] Dan [Fogelman]. Dan is always up to those tricks," Moore told ET, recalling her reaction after first coming across the scene in the script. 

Though Moore obviously couldn't divulge details, the 34-year-old actress hinted that Alan is someone from Rebecca's past.

"I think Rebecca was clearly surprised when she opened the door. I don't think she was expecting him, so that hints to me that it's not a current beau," Moore hypothesized. "And I don't think she's the type of girl who would go out on a date with another gentleman if she was currently with somebody. They seem to know each other very well if he felt obliged to step in front for a kiss and he brought flowers, so we'll see!"

NBC

Co-showrunner Isaac Aptaker confirmed that Alan "is definitely not a stranger who showed up with flowers and she's kissing." "It's obviously somebody that she's familiar with and somebody from her past. It's safe to say that she wasn't expecting to see in that moment and we'll learn more about him very soon," co-showrunner Elizabeth Berger emphasized.

There was so much secrecy surrounding the introduction of Parrish's character that Aptaker revealed the original version of the episode's opening credits, where the cast and guest stars' names are shown onscreen, had his name included. The producers ultimately nixed the idea, with Parrish's permission, in order to keep the secret as long as possible.

"We didn't want people watching going, 'Where's Hunter Parrish?'" Aptaker explained. "We put him in the end [credits] to protect that it was him. We didn't announce it. His team was very cool about letting it be under wraps so we wouldn't spoil it."

As for how this wrench will affect the beginning of Jack and Rebecca's love story, the duo warned that their journey is rockier than expected.

"We're really excited to see these early chapters of their love story that we've never seen before. They're such an incredible couple and they have this incredible chemistry that you could assume that it was smooth sailing from moment one," Berger said. "That's not usually the way life goes. We wanted to share that these two had some bumps to overcome as so many people do before ultimately ending up together."

"It's an interesting flip on your traditional romantic comedy, where we know the end of their story but we don't know the journey, and the journey is so exciting and interesting to us," Aptaker said. "We love seeing how unexpected and different ways life takes them before they wind up starting a family together."

This Is Us airs Tuesdays at 9 p.m. ET/PT on NBC.

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