What a relief. After a splurge of jetted-in celebrity casting, here are two new plays buoyed up by on-the-spot, on-the-button stage actors. Every Mike Bartlett drama requires a particular dexterity from its performers. Think of Ben Whishaw’s intricacy in the first production of Cock (2009), or Tim Pigott-Smith’s satiric skill with iambic pentameter in King Charles III (2014). In Unicorn, a terrific trio of actors – Erin Doherty, Stephen Mangan and Nicola Walker – are required to hover between lightness and pain. They have to make an explosive proposition seem not only plausible but promising. They need to register, with minimal action, considerable change in the course of two and a half hours. They rise to the challenge.