DON KNOTTS is not really nervous. But it's hard to avoid picturing him that way, since his jittery, jumpy screen persona made him one of television's most recognizable character actors.

"I wrote that 'nervous guy' character into a monologue early on in my career," Knotts says on the phone with nary a stutter or a stammer. "See, I attended this banquet and the guy who was speaking was kind of shaking all over and spilling his water, and it struck me funny. Steve Allen liked it, and made the nervous guy one of the regular 'Man on the Street' people on his 'Tonight' show.

"I think you always evolve a character with the writers of the series," says Knotts, who soon begins taping his fourth season as Ralph Furley on the ABC sitcom "Three's Company." "Of course, they always draw from things they've seen you do, or things in your own nature. For Furley, I draw a little from Barney Fife and from some of my other characters."

Developing the character of Barney Fife, Mayberry, N.C.'s, inept, hypertense deputy sheriff, "required a lot of creativity," Knotts says. "He was a very childlike man who, when he felt a certain way, couldn't hide it. He had a low boiling point, sometimes he would be blustery, oftentimes terrified.

"The 'Andy Griffith' cast was very much like a family, very much like people in a real small town," says Knotts, who won five Emmy Awards for his portrayal of Fife during his five-season tenure in Mayberry.

"I started in show biz as a kid, as a ventriloquist," says Knotts, who left Morgantown, W.Va., for Manhattan at age 17. "I just copied Edgar Bergen, to tell you the truth. It was a way of getting started." From 1951 to 1954, Knotts juggled parts in a radio drama and a children's TV show and played the neurotic Wilbur Peterson on the afternoon soap "Search for Tomorrow." "Mostly I did comedy or character roles. And a lot of stand-up comedy. I went into special service with the Army, and they put me in an Army show for two years."

Knotts and Griffith first met on the Broadway stage. "I had two small parts in 'No Time for Sergeants,' which Andy was starring in. A few years later, I saw the pilot of Andy's show, and I noticed he didn't have a deputy. Well, I was between jobs then, so I called him up and suggested it. He thought it was a good idea, and the rest is television history."

After Mayberry, Knotts parlayed his maddening milquetoast character into a series of 17 feature films to date, with titles like "The Ghost and Mr. Chicken," "The Incredible Mr. Limpet," "The Reluctant Astronaut" and "The Shakiest Gun in the West," in which Knotts finally overcomes his cowardice and becomes a hero. His star waned, he says, "about the time 'How to Frame a Figg' came out. Family films suddenly went out of popularity."

Knotts, who turned 58 last week, lives with his second wife, Loralee, in Studio City, Calif., and frequently travels to Hawaii to play golf. He has two children by his first wife: Tom, 24, an electrical engineer, and Karen, 26, an actress who "pops up on TV every once in a while, mostly in comedy."

"Andy's going to have a kind of a reunion party at the end of the month at his house. A lot of the Mayberry regulars are going to come. Ronny Howard's going to come, maybe Jim Nabors," Knotts says, excitedly. And Andy and I have been seriously talking about getting together again, for a feature film."