Neil Patrick Harris has been dishing on his brief working relation with Britney Spears on How I Met Your Mother.

After previously revealing that his guest star was “very professional” on the set of the quirky sitcom, the actor says he was surprised at how reserved the troubled singer was during her new acting gig.

He is quoted by OK! as saying: "When she was with us, she was rather shy and intimidated.

"Probably because she doesn't have a lot of acting experience, especially sitcom experience."

Responding to gossip that Brit was a ‘nightmare’ to work with, he adds: "I wouldn't call her a nightmare. She's living one."

Brit plays a doctor’s receptionist called Abby who becomes obsessed with lead character Ted in an episode to air in the U.S. on March 24.