Grant and Ms Khan stayed for barely half an hour after the episode.
In contrast, Firth and Zellweger seemed happy and relaxed.
Zellweger, 35, who has slimmed down dramatically from her Bridget Jones size 12 to a size six, even hinted that marriage might be on the cards for her and boyfriend Jack White of the White Stripes.
Asked what she wanted for Christmas following rumours that the couple are planning a seasonal engagement party, she said: "I know what I want and what I'm getting."
Firth said of the movie: "Tonight was the first time I'd seen it and I'm very happy with it. I just hope the fans are too."
Of Grant's claims that he hopes to quit acting, he joked: "All we can do is hope and pray."