You can imagine the casting conversation down at the Old Vic theatre in London when they decided to reprise Duncan Macmillan's play Lungs: a two-hander featuring a right-on young couple thinking about settling down...
Senior Creative [SC]: So, we're after a box office pairing the public would love (pay) to see reunited.
Junior Creative [JC]: What about Kate Winslet and Leonardo DiCaprio?
SC: Great idea! [pauses to think] A bit old, maybe?
JC: Okay, how about Letitia Wright and Daniel Kaluuya?
SC: Another awesome suggestion! But is she old enough?
JC: Ooh, ooh, ooh… I've got it! This is brilliant!
SC: Pray tell.
JC: Claire Foy and Matt Smith!
SC: Perfect! You. Are. A. Genius! Book 'em!
And so it came pass - probably not quite like that - the two British actors who formed a dream team double-act in The Crown for Netflix as The Queen and Prince Philip were reunited to play another young couple trying to work out their place in the world.
This time around tiaras and Buckingham Palace have been traded-in for trainers and IKEA, but they are essentially dealing with the same issues of love, commitment, betrayal, duty, compromise and existential anxiety.
There's none of the expensive paraphernalia that came with The Crown such as lavish sets and a large supporting cast.
In Lungs the stage is almost bare; the actors don't have a prop to call their own. It is entirely down to their talents to bring to life Duncan Macmillan's words in an 80-minute play in which they are constantly on the stage bantering to-and-fro without an interval to catch their breath.
It's a tall order, made slightly easier by the sheer quality and directness of the writing and their palpable stage chemistry.
Foy is superb as the doubting yet strident left-leaning intellectual with a PhD who is at once perceptive and blindly self-absorbed.
Smith does what he did as Prince Philip in The Crown, which is to play Foy's foil. Here, he is a struggling musician intimidated by his partner's intelligence and rhetorical ferocity. The full force of which is evident in the opening exchange caused by his unwitting decision to wonder aloud if they should have a child together.
She is staggered by his thoughtlessness, impudence, and lazy arrogance.
"It's like you punch me in the face and then asked me a maths question" is one of the many ways she describes the effect of his casual conversation opener while they queued in IKEA. He tries to put the pin back in the grenade but it's too late. Before he knows it she is telling him that his predatory countenance when they are in the throes of passion freaks her out, "Sometimes it looks like you are going to hack off my limbs and bury me in the woods."
He tries back-peddling, and then justifying, and eventually - when all else fails - agreeing.
It's like watching a boxing match in which one fighter is clearly stronger and more assertive while the other ducks and dives and seeks a way out by fair means or foul.
Into this semi-comic world of domestic disharmony Macmillan introduces the underlying theme of his decade-old play (first professionally staged in 2011), which is the negative impact we gas-guzzling humans are having on the planet.
Foy's character wants to know if she and he can still be "good people" if they decide to have a child, which she says will have a lifetime carbon footprint amounting to 10,000 tonnes of CO2, "That's the weight of the Eiffel Tower. I'd be giving birth to the Eiffel Tower."
It's a great line from which you can extrapolate the bigger question being asked: can we in the wealthy West ever be "good" when our privilege is at the expense of others and the planet? It is a subject that deeply troubles the playwright who wrote this "end of days" play in a single night having put aside a more complex concept.
It is a good piece of work.
But unlike his excellent subsequent plays like Every Brilliant Thing and People, Places, Things - which deal with depression and addiction respectively - Lungs runs out of breath about two-thirds of the way through.
The witty repartee between Foy and Smith pales, the unevenness of their relationship loses credibility.
That said, it is a bold and invigorating idea to focus their entire relationship on the single issue of procreation in the form of a discussion taking place over years but presented as one seamless conversation (a time-shifting exercise beautifully executed by director Matthew Warchus).
The upside for Macmillan is it allows him to highlight what he considers to be the "thing that makes drama interesting", which is, "present-tense decision-making." The downside is it ends up leaving the characters boxed in and the story with nowhere to go.
Foy's character gets bigger but predictable, Smith's smaller and boring.
The piece eventually peters out.
But not before landing some heavy blows.
Lungs turns the highly personal - deciding to have a child - into the powerfully political: it lays the issue of our age at our door. And it does so with biting wit, a sense of urgency and an appropriate level of high anxiety, all expertly delivered by the two actors.
More Claire Foy and Matt Smith combos please.
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