How true this is. Which is why we are writers, and not earning our livings on television. If I was as funny as Paul Merton, I'd be doing his job and not mine. (Mark you, I'd love to see him try to make jewelry.) It's common for me to realize what I should have said hours later. Not even l'esprit d'escalier, more like l'esprit de really, seriously, far too late. It's why I hate the phone (if you want me to agree to something I don't want to do, ring me up and ask), am happier face to face, and entirely relaxed via email.
My characters don't have this problem. They are articulate and sometimes amusing whenever they need a timely riposte. They have the benefit of me toiling over their dialog behind the scenes. Just another of the ways fiction has the edge on life.