There is no script to each episode, just a heavily detailed plot outline, from which the actors all improvise. The creators meet with the actor/actress, explain to him/her the character, and together come up with funny things they can do with it. Then they meet up one day, usually on the weekends, and shoot all the storyline in a few hours, sometimes filming up to 20 minutes improvisations.
The only main cast member from Friends not to appear on the show is Jennifer Aniston. In an interview with Vulture given in Summer 2014, Lisa Kudrow said that she had approached Aniston with the idea of doing a guest spot on the show and the actress had agreed, but then the show ended in Season 4 so the cameo never came to be.