After Blessed fainted and fell off a raised platform, Noel White, playing the Earl of Kent, said, "This is not part of the play," and asked for a doctor.
Blessed returned to the stage after 20 minutes and completed the performance.
"Brian was an inspirational wonder. He had a small heart operation three years ago," Guildford Shakespeare Company director Caroline Devlin told the BBC. "We are not entirely sure why he became so dizzy but he is a 20 year old in a 78 year old's body. There was no stopping him."
The BBC reports Blessed said, "I feel such an idiot and am not in the habit of doing this, but I have a little fibrillation."
Blessed's stage appearances include Cats and Chitty Chitty Bang Bang, among others. His film credits include "Much Ado About Nothing," "Henry V," "Hamlet" and "As You Like It." His screen work also includes "The Avengers" and "Flash Gordon."