When you say that the boy was kicking a ball. You mean he kicked the ball again and again. When you say that the bus was stopping. You mean the bus started decreasing the speed to cease to move. Then when you say that he was telling a lie. What do you mean.
This form (imperfect tense) implies that the action is still in progress when described - so if you say "at 10:45 he was telling a lie" then it's one lie, in progress when described. Equally, the boy kicking the ball may have been kicking it when he broke his toe.... he didn't necessarily do it again!
If you said it as "all day last Tuesday he was telling a lie" then it would suggest a big lie (!) or one repeated all day, as @anongoodnurse suggested - plural lies would be used for the untruthful day if he came out with a load of them!