One of the first axioms a student of theater is taught is that, to quote one professor, "theater is temporal in nature." That means a performance exists only while it is happening; there is no record of it, no replay, nor is any repeat exactly like the one that went before it.

"The Lunts: A Life in the Theater," a PBS special out of Milwaukee that will be shown at 9 tonight on Channel 26, reminds one of that basic characteristic. The Lunts, who were the premiere Interlude' that was extraneous," she says, quite rightly. O'neil refused to let her cut a word. ". . . then I cut an awful lot of it without his even knowing it," she says.

She also confesses that the only relief she felt when her husband Alfred Lunt died in 1977 at age 84 was that she could finally tell the truth about her age. It seems that she cut five yers off her age when he first asked how old she was; he cut one off his, but was later found out. Since he never asked her, she says, she just kept lying.

"I had to lie all over the place," she says. "It was terribly diffiuclt."

She looks like a regal grande dame from another era, but when she throws her head back and starts to sing a cockney music hall song, the ham in her is wonderfully on view. Wearing a dress she made herself, she hops at one point the back; "Do you like it?" she asks Schaefer.

The Lunts were married May 26, 1922, and stayed together for 55 years and countless plays. They did only two films (Fontanne is reported to have said, Schaefer relates, that they "could be bought, but we cannot be bored,") and a few television dramas, including "The Magnificent Yankee," which Schaefer directed and which won both Lunts an Emmy.

Asked for her advice to young actors, Fontanne says "if you're a success and offered a chance to go on the road, go, because if you don't, you don't have any audience." Also, she says, "I shouldn't drink too much. . . "

Most of Fontanne's performances are lost to everyone except those who had the good fortune to see them, but at least this film remains as a resource as well as a reminder.