As the 20th anniversary of the hit TV comedy which defined the 1990s draws close, three of the six cast-members of Friends appeared in a reunion which has stunned fans.

Former Friends star Jennifer Aniston was appearing on the Jimmy Kimmel Live program when the host, and self-confessed Friends fan, asked her to participate in a skit performing a piece of scripted "fan fiction" he had written.

But as the skit began the pair were joined by two other former Friends stars, Aniston's close friends Courtney Cox and Lisa Kudrow.

The skit was performed on an authentic re-creation of the apartment the three girls shared in the TV series.

Aniston was appearing on the program to promote her new film, Life of Crime.

"I loved the show so much I actually wrote some fan fiction," Kimmel said. "I wrote myself into it as Ross. I was wondering if you would do a scene from it with me?"

Aniston appeared a little unwilling to participate at first; as she walked onto the set, she exclaimed "What is this?"

When presented with a wig of the "Rachel" haircut she wore in the series, which Kimmel wanted her to put on, Aniston threw the wig into the audience.

"No, no, no, no," she said. "I am not doing that. I have boundaries, Jimmy."

It is unlikely, however, that Aniston was a genuinely unwilling participant. Her close friendship with Cox and Kudrow is well known.

Kimmel said the re-creation of the Friends set cost $US80,000.

Since the show finished in 2004, there have been persistent calls for a reunion project.

The reunion of the three Friends co-stars comes as Warner Bros unveiled plans to mark the 20th anniversary of the launch of the series with a pop-up Central Perk coffee house in New York.

The fictional Central Perk was the coffee house where the show's six characters - Rachel (Aniston), Monica (Cox), Phoebe (Kudrow), Joey (Matt LeBlanc), Chandler (Matthew Perry) and Ross (David Schwimmer) - hung out.