Can an investigator for the defendants attorney call the plaintiff and ask questions before it goes to court?

Actually the Defendant's attorney can call for a deposition. I don't believe the investigator can, but the Defendant's attorney can.

Assault with a deadly weapon is a criminal offense. In a criminal trial, there is no "plaintiff", there is the defendant and the prosecutor. In such a case the investigator is a law enforcement officer.

If this is a civil suit where someone is being sued for damages, the facts should have come out in the criminal trial. If there was no criminal trial because of a lack of evidence, then it may be hard to prove a case.

However, an investigator for the defendant has every right to ask questions of anyone to prepare their defense. Just as anyone they talk to has the right to refuse to answer or to talk to them. The only time they can be forced to talk is when under oath at a trial or deposition.