Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matt LeBlanc, Matthew Perry and Schwimmer will all feature in the special.
Schwimmer, who played palaeontologist Ross Geller in the much-loved sitcom, told The Graham Norton Show: “I’m going to LA to shoot the Friends reunion. I will get to see everyone for the first time in years.”
The actor confirmed the cast will appear as themselves and not as their alter-egos.
“Nothing is scripted and we are not in character,” he told Norton.
“We are all ourselves, although there is one section of it that I don’t want to give away, but we all read something.”
He quipped: “I should have genned up on old episodes, but I just haven’t been able to bring myself to catch up on the 236 shows. I guess I need to watch a lot over the next five days!”
Norton’s show also features Hollywood stars Octavia Spencer and Melissa McCarthy, British actor Nick Mohammed, comic Frank Skinner, and RuPaul’s Drag Race star Michelle Visage, who performs with Steps.
Mohammed says GCHQ was supportive of his spy sitcom Intelligence, which also stars Schwimmer.
“They did get in contact with us when they heard we were doing a show about GCHQ, but when they realised we wanted to know more about their canteen, choir and bake sales and that we were making a silly sitcom and not a whistleblowing drama, they relaxed a bit and were incredibly supportive,” he said.
The Graham Norton Show airs on BBC One tonight at 10.45pm, and on Virgin Media One tomorrow at 9.35pm