Despite CBS hype about how great it was working with BRITNEY SPEARS on "How I Met Your Mother," she didn’t see eye to eye with her costars – at first, anyway – because Ditzy Diva wore her humongous, dark sunglasses during rehearsals!

"Britney delivered her lines, but the show’s actors – JOSH RADNOR, NEIL PATRICK HARRIS and guest star SARAH CHALKE from ‘Scrubs’ – had a tough time reading her expressions since they couldn’t see her eyes at all," said a source.

"It’s hard for an actor to make a connection when all he sees is his own reflection staring back at him in shiny dark glasses…but no one wanted to upset Britney by asking her to take them off." Finally, a CBS exec whispered nervously to Britney: "You will be taking the sunglasses off for the actual tapings, right?" Staring back through her movie star shades, Brit airily replied: "Uh…yeah!"