A day of "anarchy" at Radio 4 will feature the return of The Vicar of Dibley, The Hitchhiker's Guide to the Galaxy and Sesame Street's Big Bird.
Character Invasion will see the BBC radio station taken over by a host of well-loved fictional creations on Saturday 29 March.
Dawn French will reprise her role as village cleric Geraldine Grainger to deliver a special Thought for the Day, written by Dibley creator Richard Curtis, on the Today programme.
French made her last screen appearance as Geraldine in the final episode of the series in 2007 but just last year she made a comeback in character on Radio 2, chatting to Chris Evans on his Pause for Thought section.
Naturalist Chris Packham will be reporting from the streets of New York for Tweet for the Day, hot on the trail of Avis Giganteus – more commonly known as Big Bird, whose natural habitat is Sesame Street.
Douglas Adams' creation Marvin the Paranoid Android will be among the other characters storming the studios and taking over the airwaves, while the rest of the cast of The Hitchhiker's Guide to the Galaxy – led by Simon Jones as Arthur Dent – will reunite to perform highlights from Adams' beloved sci-fi comedy.
A new production of Hamlet starring History Boy Jamie Parker will be broadcast over five afternoons, concluding on Character Invasion Day, while double Booker Prize winner Hilary Mantel and Bridget Jones author Helen Fielding are set to discuss what it's like to final let go of characters they've created.
"I am delighted to announce anarchy on Radio 4," said Radio 4 controller Gwyneth Williams. "Some of our favourite characters from drama, literature and comedy will be dropping in and out of the schedule when you least expect it to explore and examine the nature of character.”