The Lakers seem to play their best basketball when they're on the run and executing their fast break like they did in the third game on Sunday. But when they're standing still - as in on the free-throw line - it was an entirely differcnt matter tonight. The Lakers, who led by 2 in overtime, 123- 121, when Worthy, who finished with career-playgff high 27 points, scored a 3-point play with 1:36 left, missed 4 of 9 shots from the line in the extra period.

The most critical were by Johnson, who banged two off the back rim with the score tied, 123-123, with 35 seconds left. After his miss, Johnson was victimized on the other end when Bird, who had been covered throughout much of the night by Micheal Cooper. On the play, Bird lost his man on a swicth and found himself being guarded by Johnson.

''When I saw that, I tried xo post him up as quick as I could,'' Bird said. ''It was kind of helter skelter then but Dennis Johnson got me the ball and I knew I could shoot over Magic better than I could over Cooper.''

Dennis Johnson, who has been maligned for his poor play in the series, had his best effort with 22 points and 14 assists. Johnson also played Magic Johnson on defense for the first time in the series, which was a reason why the Lakers scored only 2 fast-break baskets in the second half before overtime.

''Everybody came out and did their jobs tonight,'' said Boston forward Cedric Maxwell.

After Bird's basket made the score 125-123, Worthy made one of two free throws with 10 seconds left. Then Dennis Johnson was fouled on the inbound play before any time expired and hit two free throws to put the Celtics ahead by 127-124.

With the Lakers trying desperately to set up a 3-point shot, M. L. Carr stole Worthy's inbound pass near midcourt and sailed in uncontested for a dunk to put the Celtics up by 129- 124 with 7 seconds remaining. Bob McAdoo's free throw with 3 seconds left finished the scoring. Format May Change

David J. Stern, the league commissioner, attended the game and said that he's asking the Rules and Competition committee to study the feasibility of changing the championship series from the current 2-2-1-1-1 format to one that calls for one similir to baseball in which the first 2 games are played in one city and then the next 3 in the other home arena.