Robert Brown played both versions of Lazarus, the tormented traveller and his calm antimatter counterpart, in The Alternative Factor, taking over the part at very short notice.
Robert Brown (1926–2022) is best known for the series Here Come the Brides, but he also took the central guest role in the first-season episode The Alternative Factor.
He was a last-minute replacement. John Drew Barrymore had been engaged to play Lazarus and failed to report for filming, and Brown stepped in with almost no time to prepare.
The part is a double one. One Lazarus is a raving, vengeful man hunting the being he says destroyed his civilisation; the other, from an antimatter universe, is rational and knows that a meeting of the two outside the corridor between universes would annihilate both. Kirk finally traps them together in the corridor to fight for eternity.