It is not clear to me what it means to “paint nothing in particular happening”. Maybe the author means to paint some scene in which there is very little action — but painters paint such scenes all the time (e.g., still life paintings, countryside paintings), so maybe the author means something else. “Without overdoing it” means “without going to too great an extreme”. Examples of use of “over do”: “I really overdid it at the gym yesterday, so my muscles are really sore today.” “That woman can just say a simple thank you. She always overdoes it and acts so extremely grateful that it makes me feel uncomfortable.” Direction to an actor by a director: “I want you to show more emotion, but without overdoing it. Don’t start crying, but I want to hear the emotion in your voice.”