Los Angeles — The annual National Basketball Association playoffs, now under way with eight teams battling for four quarterfinal berths, are always a bit of a mystery in terms of what to expect.
The reason is that so often pre-season favorites wind up eating dust; there is usually a Cinderella or two in the group; and centers get written about with almost as much in-depth analysis as the Washington press gives President Reagan's cabinet.
''I think the biggest change between the regular season and the playoffs is in the quality of everybody's defense,'' said head coach Pat Riley of the Pacific Division champion Los Angeles Lakers. ''Most players become so aggressive when the other team gets the ball that they turn the playoffs into a halfcourt game.
''Teams can survive if they execute well,'' Riley continued. ''But if they expect to get the same amount of penetration in the playoffs with the same effort they put out in the regular season, they aren't going to get very far. It's like two different seasons, with the second one making it a lot tougher for running teams.''
For a veteran player's viewpoint we went to Los Angeles guard Norman Nixon, who led the Lakers in minutes played while making the Western Conference all-star squad.
''During the regular season, players have a tendency to pace themselves,'' Nixon explained. ''While you might like to go all out at both ends of the court, the constant travel across time zones combined with our 82-game schedule simply doesn't permit it. Anytime you play four games in five nights you know it physically.''
''But everybody's game gets better in the playoffs because there isn't anything to save up for anymore. Now you're not playing to get through a long season, but for a championship, and it makes a difference. It's amazing the way your energy goes up in the playoffs and stays there.''
The four division winners -- Boston, Milwaukee, Los Angeles, and San Antonio -- have byes in the current best-of-three preliminary series. Squaring off, meanwhile, are Washington and New Jersey (the winner to meet Boston); Philadelphia and Atlanta (the victor to play Milwaukee); Denver and Phoenix (for the right to go up against Los Angeles); and Seattle and Houston (the survivor to meet San Antonio).
Because he played four years for the Celtics in the mid 1970s and is familiar with their system, head coach Paul Silas of the San Diego Clippers was interviewed about Boston's playoff chances. He was asked specifically what Eastern Conference teams might be the biggest stumbling blocks for the defending NBA champions.
''The Celtics have such a deep bench, play such good defense, and don't rely on any one shooter for their offense, that it would take a terrific effort by any team to beat them in the playoffs,''Silas said. ''Boston has been through so many pressure playoff situations that its personnel should be able to handle almost anything that comes up.''
''Among Eastern Conference teams, I think Philadelphia would give Boston more problems than Milwaukee, mostly because the 76ers have done it before and because you know that Dr. J. (Julius Erving) is going to get his points,'' Paul continued. ''Philadelphia also has two centers it can rotate against Boston in Darryl Dawkins and Caldwell Jones, plus a guard that can provide instant points off the bench in Andrew Toney.
''The trouble with Milwaukee is that it already has two regulars (Junior Bridgeman and Quinn Buckner) out with injuries. To win the Bucks would also have to get a lot of extra playing time from center Bob Lanier, who at this stage in his career doesn't bounce back the way he did when he was younger.''
Assuming that Boston were to reach the finals and that either Los Angeles or Seattle prevailed in the West, which of these teams did Silas think would provide the Celtics with their toughest test?
''For Los Angeles to beat Boston, two things would have to happen,''Paul replied. ''Kareem Abdul-Jabbar would have to play enough minutes to nullify the combined efforts of Celtic centers Robert Parish and Kevin McHale. That is, if Coach Bill Fitch decided to use McHale at center as well as at forward. And Michael Cooper, who guarded Larry Bird so well in the two regular season games the clubs split, would now have to do that for an entire series, which might not be possible.
''Of course, if Kareem wasn't able to handle Parish or McHale for any reason, then the Lakers wouldn't have a chance anyway,'' Silas added. ''What it comes down to is that L.A. doesn't have a backup center strong enough to stop Boston defensively. While Bob McAdoo would probably be all right offensively, Parish and McHale would overpower him at the other end of the floor.''
And if Boston were to play Seattle?
''As for the Sonics, I don't think their forwards are good enough after Lonnie Shelton to contain Bird or McHale or Cedric Maxwell,'' Silas said.
''I like McHale because he is a strong rebounder who does the same kind of physical things close to the basket that I used to do, only he does them better, '' Silas observed. ''Considering all the angles, the team you can never discount is Boston.''