"It feels so great to be reunited," MacGraw tells PEOPLE. "Our chemistry has not changed. We still feel the same about each other."
Agrees O'Neal: "We are more than friends. She is like my sister, only she had better lock her door at night so I can't get in there. We have some unfinished business, and we are running out of time!"
The actors play a couple from similar backgrounds who meet while they are young but end up taking different paths in life. Throughout a 50-year friendship, marriages and families with other people, they can't let go of their bond.
The week-long theatrical event starts Tuesday at Fort Lauderdale's Broward Center for the Performing Arts. The production will then travel around the U.S. to Beverly Hills, Detroit, Boston, Dallas, Baltimore and more cities yet to be named.
"Ryan is so funny, so good and he is such a star, that I am terrified and excited to be reunited with him on stage," MacGraw, 76, says. "He is really sensitive even though he can be perceived as macho."
O'Neal, 74, believes it was fate that brought them together to do the play: "Working together again is a dream come true," the actor says. "Ali is an original. She is giving and sensitive to others, and we have never had an argument or cross word all of these years."
RELATED VIDEO: Tatum O'Neal Reveals She Is Dating Women
"It is always an adventure with Ali," adds O'Neal. "It is so nice to re-discover her because I have had such a crush on her!"