The Phoebe Buffay actor has also revealed filming on the unscripted special is set to begin in the “early spring” after a series of delays.
It was announced last year that Lisa and her fellow co-stars David Schwimmer, Jennifer Aniston, Courteney Cox, Matt LeBlanc and Matthew Perry would be reuniting to celebrate 25 years of the hit US sitcom.
The special episode had been set to air last May, but has been pushed back twice due to the coronavirus pandemic.
Speaking on Rob Lowe’s podcast Literally!, Lisa said: “I pre-shot something for it already so we’re definitely doing it, because I already shot a little something.”
Describing the format of the reunion, she said: “It is not a reboot, it is not like a scripted thing, we are not portraying our characters.
“It is us getting together, which just doesn’t happen and has never happened in front of other people since 2004 when we stopped.”
She added the special will feature the cast “talking in a coffee house room” and “some sets will be up”.
“I think it will be great,” she added.
Friends ran for 10 series from 1994 and remains one of the most popular shows on TV.