United Kingdom, Jan. 27 -- Kelly Clarkson has apologised to the cast and crew of 'The Rookie' after making a cameo appearance on the show.

The 42-year-old pop star previously landed a walk-on role on the TV drama series, and Kelly has now confessed to feeling "terrified" throughout the shoot.

The singer said on 'The Kelly Clarkson Show': "Oh my God, I was terrified. I have to apologise to that cast and crew. I was terrified.

"'The Rookie', they invited me, and here's the thing - I'm a huge, like, I've been a fan of Nathan Fillion since 'Castle'. I already get nervous on sets, but then he made me more nervous than I have ever been on a set ... I was an idiot on that set."

Kelly would love to appear on 'Only Murders in the Building', th...