The Downton Abbey star appeared opposite the Hollywood actress in director Sam Mendes' 1998 production of David Hare's play, in which they both shed their clothes.
Glen, who played dastardly newspaper magnate Sir Richard Carlisle in Downton Abbey, has now revealed he grew very close to Kidman during their theatre stint, but they have since drifted apart.
He says, "It was one of those dream jobs... We were going to be working together, intimately, for a long time. It was important that we got on. I needn't have worried. I found her so focused and rather brilliant as an actress. We had a very happy rehearsal and then it turned into this phenomenon...
"There was I, cartwheeling, doing handstands and playing the piano without a stitch of clothing to little or no reaction. And yet, when Nicole briefly slipped out of her dress, her back to the audience and showing only the hint of a buttock, you could feel a frisson run around the theatre. We stayed good friends for about two or three years but life moves on."