Married to her alcoholic agent (Michael Landes), she, her sister Miranda (Gemma Whelan) and their families (James Fox, Sally Phillips and Ronni Ancona are all part of the dysfunctional mix) reunite in their deceased parents' dilapidated country house for a three-day Christmas ordeal.
Breaking off from an account of filming with Fatal Attraction director James Dearden, who based the script on manic Christmases of his own, she makes the poignant aside: "I don't know why, but I've been thinking a lot of my father recently."
These conversational tangents, betraying a restless mind cluttered with literary references, are another reason you warm to Richardson. But it's the idea she needs to explain what led her to make her "happiness vow" that's perhaps most touching. Because as the sister of the late Natasha Richardson, who died in a skiing accident in 2009, hers is a story and a sadness we've followed for years.
"Losing my sister meant that there were many years of grief and those years were tough," she says quietly. "Because it wasn't just three months or six months, it was a great big chunk of time. And it wasn't just my loss, it was my mum's and her boys' loss."
She doesn't want to go back over the pain or see another headline about her grief, she adds with a small smile, "because now I'd sort of like to see a different headline. And, yes, there were other losses around the same time - some of my best friends died. So for a few years I just thought: 'God!"'
Closing her eyes for a second, Richardson rakes her hands through her thick blonde hair: " 'I didn't think life was going to be like this!' And there was an element of just surviving. But then I turned 50 and had this reaction. Because we have this window, don't we?"
Her sister's death and the near-fatal heart attack her mother suffered in 2015 may have heightened Richardson's awareness of this opportunity to wring joy, but the actress has always come across as someone who lives fully, vigorously, in her private and professional life.
She may have thrown herself into work after her marriage to Working Title co-founder Tim Bevan ended (they have a daughter, Daisy, 26, also an actress), and swung from the hit US TV drama Nip/Tuck and one-woman shows to blockbuster movies such as Red Sparrow, but she found time for relationships with high-profile men such as Jamie Theakston and Russian businessman Evgeny Lebedev, and others she won't discuss.
All she'll tell me today is that she recently split from someone. "So this Christmas is a bit of a 'wait and see' job, but it'll probably be a family one in London because Mum's doing a play here. And I've got the builders in."
The "petty fights, strange shenanigans and logistical issues involving turkeys" that crop up in Surviving Christmas with the Relatives are something Richardson can relate to. But the curious sadness many feel at this time, at the passing of another year? "No. I don't feel that. Yes, time becomes more precious and the choices you make more essential, but it goes back to me wanting to enjoy the time I have. And I'm not going to say that because I'm over 50 and a woman, I'm no longer desirable," she adds with an eye roll.
"All of that is surface stuff. And the great thing about getting older is that it becomes about a person's humanity, not whether you've 'had stuff done' or are ageing gracefully. I might get Alzheimer's or dementia and have no choice in matters of 'grace'.
"So, honestly, I'm very pro 50s, because, above all, you get what I would describe as a surety of purpose."
In the new year, she'll star alongside her mother in an adaptation of Henry James' The Aspern Papers. She's also in a new TV series, The Rook. And when people try to commiserate over "how tough it must be getting roles when you're older", she throws it back at them. "Things have only got more interesting for me! And if I'm lucky I can keep going into my 80s, like my mum."
