Oscar-nominated Irish actress and singer Jessie Buckley has been announced as an additional cast member for Tim Crouch's play An Oak Tree.

The play, performed at The Young Vic Theatre in London, has been given an extended run of May 6 to May 24 in celebration of its 20th anniversary.

Other guest performers include Adjoa Andoh, Mark Gatiss, Arthur Darvill, Julie Hesmondhalgh and Alex Lawther. Each night, the guest performer is not revealed until they appear on stage for the play.

An Oak Tree, written and performed by Tim Crouch, uses the concept of theatrical "cold-reading", where each guest actor performs without reading or seeing the play beforehand, and will only perform it once.

The play tells the story of a father whose daughter dies in a car accident. The driver, a stage hypnotist, meets the father when he volunteers one night for the hypnotist's act.

"Exactly twenty years ago a laboratory was created in a small room above a pub in South London where an experiment took place," said Crouch. "Actors were invited to try out a new play I'd written called An Oak Tree - like trying out a new drug formulation."

"Each time a different actor who knew nothing about the play they were going to be in - a blind test each time."

Since its 2005 debut, An Oak Tree has been staged across 19 countries with over 350 guest performers.