Chisnall currently understudies Lindsay, and has also played Farquaad's henchman Thelonius and Papa Ogre. Previous West End credits include Love Never Dies, La Cage aux Folles, Never Forget, Evita and The Woman in White.
McDermott is best known for playing Ryan Malloy in TV's "EastEnders." He has also appeared onstage in La Cage aux Folles at the Menier Chocolate Factory and The Sound of Music at the London Palladium.
Further casting is still to be announced.
To book tickets, contact the box office on 0844 871 8810, or visit www.ShrekTheMusical.co.uk.