JaneV has found an interview with the actress who plays Sybil in Private Lives, Lisa Dillon. In the interview, Lisa has the following comments:

"Sometimes Kim and Matthew will run their dance sequence or practice their song." and

"Matthew is delightful, incredibly warm and good fun. He’s a bit of a giggler and is often found with his head buried into a cushion, trying not to laugh."

Read the full interview, as it provides a lot of insight into the practice involved.