The character Sweeney Todd originated in serialized Victorian popular fiction, known as penny dreadfuls. A story called The String of Pearls was published in a weekly magazine during the winter of 1846–47
Eventually it made it to Broadway. The musical was based on Christopher Bond's 1973 play Sweeney Todd, which introduced a psychological backstory and motivation to Todd's crimes.
When Sondheim first brought the idea for the show to director Harold Prince, his frequent collaborator, Prince was uninterested, feeling it was a simple melodrama but it could be OK with the right star.
Sondheim thought stage veteran Angela Lansbury would add some needed comedy to the grim tale as the lunatic Cockney shopkeeper, but Lansbury needed to be convinced. When asked to do it she replied, "I am a STAR. And said to Sondheim, "Your show is not called 'Nellie Lovett', it's called 'Sweeney Todd'.
What an EGO
She finally agreed to do it provided she get approval on all her songs and the final say on who would play the tortured barber. She agreed to allow Len Cariou to be on stage with her
Angela later said, "I was left to my own devices and had to reinvent the character of Mrs. Lovett and I played her cuckoo style
Len Cariou said she completely took over everything and the entire cast had to run with it her way or else