The behavior of the defendant before and after the death of the victim does often come into play in murder cases. And, you certainly don't need an expert to explain how an innocent defendant or guilty one would act. Most jurors are able to connect the dots themselves. Here are some examples of prosecutors doping exactly what you say they cannot - telling that the evidence will show the defendant acted in a manner that points to their guilt. These are only two of many cases, but just ones I could find very quickly:
"In his opening statement, Prosecutor Greg Davis told the jury, the state of Texas and the country that 'the real Darlie Routier is a self-centered woman, a materialistic woman and a woman cold enough to murder her own children.' His case was built on the premise that Ms. Routier, angry over losing her money, her freedom, and her figure, brutally and savagely stabbed her two young sons to death while they slept, then staged a crime scene and blamed an intruder."
http://www.fordarlieroutier.org/MediaArticles/Good/Good3.html
Also quoted in:
http://abcnews.go.com/TheLaw/LawPolitics/Story?id=6451238&page=1
"REDWOOD CITY, Calif. As opening statements began Tuesday in the double-murder trial of Scott Peterson, prosecutors started detailing a case they hope will prove beyond a reasonable doubt that Peterson killed his pregnant wife and unborn child.
Prosecutor Rick Distaso told jurors that Scott Peterson called his mother-in-law that afternoon and said he had returned from a fishing trip to an empty house though he allegedly later told the uncle of his wife Laci that he was golfing all day.
'He says, "Mom, Laci's missing,"' Distaso told jurors. 'Right then, Sharon Rocha knew that things were very seriously wrong.'
By nightfall, police and family were investigating a missing person report, which would unfold into the case that captivated the nation.
'It's Christmas Eve, there's a woman who's eight months pregnant who is missing under very mysterious circumstances,' Distaso told jurors, over objections from defense attorneys. 'They're looking for evidence of a burglary. They're looking for evidence of a robbery. There is nothing out of place.'...Rocha joined investigators in a panicked search of garbage cans at a fog-shrouded park near the couple's Modesto home, where Laci Peterson used to walk the family's golden retriever. Distaso described Scott Peterson as terse with family and unable to tell police what he had been trying to catch on his fishing trip to San Francisco Bay.
By Christmas Day, Peterson was more engaged and talking in ways that Distaso suggested point to his guilt. For example, he said, Peterson called police to ask if they were using cadaver-sniffing dogs to search the park. 'We haven't come to the conclusion yet that Laci Peterson is dead,' Distaso said the officer told Peterson. 'That kind of sets the stage for this entire case.'...
Prosecutors have had more than a year to prepare their case which California Attorney General Bill Lockyer called a "slam dunk" on the day authorities arrested Peterson. But the lack of direct evidence linking Peterson to the murders has turned the case from a sure thing to a spin zone of circumstantial evidence for prosecutors, legal experts say. Prosecutors don't have a murder weapon or even a cause of death.
The main focus of the case is shaping up to be how tight a web the prosecution can spin, and how ably the defense can explain away Peterson's behavior following his wife's disappearance.
On Tuesday, Judge Alfred A. Delucchi gave jurors the standard admonishment that circumstantial evidence cannot lead to a finding of guilt unless the facts 'cannot be reconciled with any other rational conclusion.'
But circumstantial evidence shouldn't be underestimated, said Bill Sullivan, a former federal prosecutor.'The jury will be directed that circumstantial evidence is just as compelling as direct evidence,' Sullivan told Fox News. 'Circumstantial or not, all this evidence can lead to a guilty verdict. It is the collection of circumstantial evidence, it is the totality ... shown time after time, example after example it will point in one direction.' Sullivan said he feels that the prosecution will be able to argue that 'Scott had the motive and the opportunity at the exclusion of all others.'"
http://www.foxnews.com/story/0,2933,121355,00.html