The last ever episode aired, to triumphant reviews, on Monday night and despite being in it, Sian has been tuning in like the rest of us.
And then rewatching it, again, like the rest of us.
“We always joked we would just do two [seasons] like The Office,” Sian laughs, speaking to HuffPost UK shortly before the finale aired on BBC One. “After filming, I kept saying to Phoebe, ‘Yeah but we’ll do a third one right?’.
“I kept nagging at her and I had it in the back of my mind that I would keep mentioning it every six months.
“But I actually feel completely at peace with it now. I know we’re going to go on to collaborate in the future but it feels so right to set her free now.”
The final installment of Fleabag hit every button possible – we laughed, cried and gasped our way through it, as one marriage began, another ended and the title character’s relationship halted before it had even started.
Claire’s first pivotal moment came as she revealed a secret we’ve been watching her keep quiet for weeks, blurting out to to family members, including her husband Martin, that the miscarriage which occurred in episode one was hers.
Sian describes this, and the marriage break-up which soon follows, as “two of the most challenging scenes” she’s ever filmed, citing a lack of time and of course, the emotional content.
“It was a high pressure situation for sure, I remember that,” Sian says. “But it was also very thrilling to be doing it.”
To make things even tougher, there were frequent rewrites, with Martin’s last ditch bid to keep his marriage alive being redrafted in the closing moments.
The conversation the cast were having was “How much we should give the man to say in that scenario?”, Sian says.
“It’s all him that gets to express himself.” she explains. “I love the scene as it is, because Claire knows what she wants at that point, she’s beyond the decision.
“She’s ready for it to be over, it is him that’s begging for mercy, it’s not on Claire.
“I think you just want that suspense of whether she will be convinced or not... and fortunately not. She’s got enough wits about her at that moment.”
In these seconds, which elicited cheers from plenty of viewers, the usually uptight Claire finally loosens her shoulders and breathes a sigh of relief.
But for Sian – who has watched the second series “tens of times” since it was added to iPlayer – the seismic change in her character comes long before the final unravelling.
“I feel like there’s a massive shift when the haircut happens,” she says, referring to the episode five incident which resulted in her favourite line of the series (‘I look like a pencil’).
”I don’t know that that was intentional but it [the haircut] creates a shift in her,” Sian continues. “Obviously she runs into Klare shortly after that, but yeah.”
And while viewers cheered, it’s unlikely any were as pleased as the actress herself, who shouts a loud “YES, finally!” at the mention of her character’s marriage coming to an end.
“I felt incredibly happy that we got that ending for her, on her terms,” she says. “That was really important. She came to it in her own way, she came to it in her own time.
“Maybe it will inspire other women to do the same,” Sian adds, perhaps half-jokingly, before continuing: “I just think it’s a very empowering move, but it’s also really realistic at how hard it is to have those reckonings with yourself.
“For Claire, failure is death. And to admit that her marriage has failed was not an option for Claire.”
Sian’s last ever scene as Claire is midway through her dad’s wedding ceremony, as shortly after telling Fleabag why she can’t run off to follow Klare to the airport, she gets up, removes her hair extensions and walks out to make that rom com-style dash.
Has Sian found herself wondering if Claire made it to the airport in time? If her on-screen alter-ego is now enjoying life with her new love, thousands of miles away from her creepy stepson and his dad?
“We didn’t talk about where that story goes, but she’s going to the airport,” Sian says. “Whether she finds him, whether she can get through security all of those things, is another matter.
“I guess the romantic in me wants to believe that she finds him and even if she doesn’t she can just hop on a plane to Finland and go get him on the other side. But nothing definitive in my mind.”
There’s one other act of love there too though, as just before we get to that final goodbye, Claire tells her sister: “The only person I’d run through the airport for is you.”
It’s one of the most tender lines of the series and while it’s not a shock to us – we’ve seen the two women rebuild their relationship in a way only sisters often can – a flicker of astonishment and emotion crosses Fleabag’s face.
It’s perhaps the most pertinent reminder of what this series was, as declared by Fleabag in episode one’s first fourth wall break: “This is a love story.”
“The real love story of Fleabag is the sisters,” Sian says. “That’s the true heart of the piece.
“I think Phoebe’s biggest hurdle to overcome when she was making a second series was how she would bring those sisters back together.
“I think after episode five, Claire knows that Fleabag is her number one, that she’s her rock. They are solid after that.
“But I don’t think that moment really lands for Fleabag until Claire says, ‘you’re the only person I’d run through the airport for’.
“Sisterhood is so special, so important and so beautifully portrayed in this. That was very important for us, to celebrate that relationship and for that love story to find its way home.”