Fresh from subbing for the late John Huston in the film "Mr. North," Robert Mitchum is coming to the rescue again: he's filling in for the recovering Edward Woodward on the CBS series "The Equalizer." The appearances (it's a two-part episode) are believed to be Mitchum's TV series debuts. According to the script, Mitchum stars as Dyson, an old friend of Robert McCall's (Woodward) and former associate in intelligence work. McCall is kidnaped and Dyson is called in to find him. The episodes will be telecast Oct. 28 and Nov. 4. Woodward, who had a mild heart attack in July, is scheduled to return to New York on Monday and begin work on the series Oct. 15.