Here is an interesting comparison between premeditated murder and a spontaneous one. I'm not sure which site I got this from.
Example of a Willful, Deliberate, Premeditated Murder
Imagine that Joannie and her husband Tim are in a terrible fight in the kitchen. Tim tells Joannie that he is going to get a divorce and will thereafter seek full custody of their two young children. Joannie states, “Wait here. I need to go to the bathroom. I will be right back.” She walks down the hall, but goes into the bedroom, rather than the bathroom, and removes a handgun from the nightstand drawer. She then walks to the bathroom and flushes the toilet. Hiding the handgun in the pocket of her bathrobe, she walks back into the kitchen, removes it, and shoots Tim four times in the abdomen, killing him.
In this scenario, Joannie probably could be convicted of premeditated murder in most jurisdictions. Joannie shoots and kills Tim in a calm, methodical manner, evidencing deliberation. Her manufactured excuse and flushing of the toilet indicate planning. The act of shooting Tim four times shows that Joannie has a specific intent to kill and a strong and calculated desire to bring about Tim’s death. Note that timing is not an issue here. Even a few minutes are enough to carry out a premeditated murder if the proper facts are present.
Example of a Spontaneous Killing
Compare the previous example with this scenario. Frank, Dillon’s supervisor, calls Dillon into his office and fires him. Enraged, Dillon grabs a heavy brass paperweight from the top of Frank’s desk and strikes him in the forehead, killing him instantly. In this example, Dillon acts in anger, not calm, cool, reflection. The act of grabbing a heavy brass paperweight appears impulsive, not planned. There is no evidence to indicate that Dillon knew he would be fired or knew that there was a brass paperweight on Frank’s desk. In addition, the single blow to the head does not necessarily indicate that Dillon had a strong and calculated desire to kill Frank. Dillon’s conduct may be supported by murder intent (most likely implied malice, knowingly, or recklessly under circumstances manifesting extreme indifference to the value of human life), but there is no evidence of specific intent to kill, deliberation, or premeditation. Thus Frank’s killing would probably not be first-degree premeditated murder in most jurisdictions