When not trading quips with David Letterman, comic Calvert DeForest (a.k.a. Larry ''Bud'' Melman), 71, can be found at home watching All My Children and One Life to Live. He's been obsessed for almost two decades: ''I can't program my VCR, so I watch every day. I also get all the soap magazines. I'm an addict.'' His favorite actors are Michael Knight (AMC's Tad ''He's so charismatic. But that twin story line left me puzzled''); Jeanne Cooper (Y&R's Kay ''I don't watch her show, but I've met her, and she is a regular doll''); and Jessica Tuck (formerly Megan on OLTL ''She was so great. I have a small role in a movie (Don't Stop Now) she's going to be in''). What about an AMC guest shot? ''Not with all that dialogue. One line I could do. Any more, and I think I would embarrass them and me!''