19 Bruce Willis guest starred in Friends because he and Matthew Perry, who co-starred with him in “The Whole Nine Yards”, thought it would be fun. He did the role for free and donated the money to charity.

Matthew Perry, who plays Chandler in the show, and Bruce Willis became great friends while filming the comedy movie “The Whole Nine Yards”. They both decided that it’d be fun for Willis to guest star in the show. So, he starred in three episodes as Paul Stevens, the father of Ross’s student and girlfriend, Elizabeth, whom he fathered when very young. Willis’ character shows a strong dislike of Elizabeth and Ross’ relationship because of the age gap and threatens to call his university if it wasn’t ended.(source)

20 The golden frame around the peephole of Monica’s apartment front door was originally a mirror which was accidentally smashed by one of the crew members. But series makers thought it looked good and decided to leave it there. 

21 The orange couch in Central Perk, the coffee house the six main characters frequent, was found in the basement of the Warner Bros. studio. 

22 Except for the crucial parts or finales, all the scenes on Friends were filmed in front of a live audience.

When the filming began in the summer or 1994, the audience were given a summary of the series to familiarize themselves with the six main characters. To entertain the audience between the takes, they also hired a comedian. It took them around six hours to film each episode of length 22 minutes, which was actually twice the amount of time most sitcom tapings take, because of the many retakes and changes in the script they had to do.(source)

23 Monica and Joey were originally intended to be the main couple of the show, but was abandoned because of Matt LeBlanc’s approach to their relationship had a “big brother vibe” rather than a romantic one. 

Monica and Joey, who were conceived as the show’s most sexual characters, were replaced by Ross and Rachel. When the writers saw the way LeBlanc played his character, they felt that Joey’s brother-toned relationship with Monica felt much better than a romantic one, causing them to prefer the former. However, the writers also made an episode “The One That Could Have Been” in which the characters imagine how their lives would be if things had worked out differently. Monica and Joey were shown to be a married couple in it.(source)

24 Ross was the first of the six to be cast. The character was written with David Schwimmer in mind.

When the auditions first started, there were around a 1,000 actors who applied and were shortlisted to 75 by the show’s casting director. However, David Schwimmer was the first to cast among all the six characters. The creators of the show, Crane and Kauffman, had worked previously with Schwimmer and wrote the character Ross Geller with him in mind.(source)