I suppose the DA determines what the call will be, initially, since he/she is prosecuting the case, based on the evidence collected. Once the trial plays out, it may become apparent that the murder was not pre meditated, the jury would rule that, and a lesser sentence of second degree, or even manslaughter, will be handed down. But usually, the degree of murder is pretty much decided on before the trial starts. All parties concerned are given the same evidence before trial so there usually/rarely (if at all) aren’t any surprises during the trial.
Don’t you ever watch Perry Mason?