Now that Rebecca's big secret is out of the bag on This Is Us, almost everyone in the family knows about her relationship with William from years ago. But I'm sure some of you may be wondering: did Jack know about William on This Is Us back when the secret was first introduced? Judging by everything that viewers have seen so far, it doesn’t seem likely that anyone but Rebecca ever had contact with William, and that goes for Jack too. It’s obvious from all of the flashbacks that Jack had a special relationship with each of his three children, so if he had known something about Randall’s birth father, then he might have tried to help Randall know about his blood relatives. But as it stands, it looks like Rebecca’s secret on This Is Us is something that she kept to herself for the past 36 years.
The big secret is that, shortly after Randall was born and Rebecca and Jack brought him home with their twins, Rebecca went to William’s house and talked to him about trying to find a connection to her adoptive son. It was at William’s apartment that Rebecca was given the book of poems that would inspire Randall’s name, so it was kind of a big deal. So much of a big deal that, apparently, William never forgot about their meeting and any other correspondence they had in the years to come.
But did Jack know about William on This Is Us? Obviously he wasn’t around in present-day when the big secret blew up over Thanksgiving dinner, but in the past, before he died, could he have known about Rebecca and William? It certainly seems like her meeting with their son’s birth father was something Rebecca kept to herself over the years. She’d made it clear that she had had no intention of allowing William to know Randall or to check in on him, so it would stand to reason that she wouldn’t find it necessary to tell anyone about meeting him, let alone Jack, who would've probably tried to convince her otherwise.
Obviously Rebecca figured that keeping the secret from Randall was for his own good, so she likely figured that doing so would be in the best interest of Jack as well. Honestly, it’s such a huge burden to bear on your own, let alone to place on someone else’s shoulders, so it’s almost understandable that she kept it to herself. Still, Rebecca’s secret on This Is Us could change her relationship with Randall forever and that's the scariest part of the reveal.