Dad died, and left me in charge. I have no idea how things work! I do have the original Last Will and Testament as well as the other legal documents that you prepared for him, but his investment account representative is requiring something from the probate court which they say would allow me to be the Personal Representative. She was calling from another state, and wanted to know if Dad's will had been "filed with the circuit court." She had also been told that Dad's estate is "greater than what a small estate affidavit would cover." The lady from the investment company said that although my dad says in his will that I am to be the Personal Representative, the court needs to approve that decision. Any light you can shed on the process would be greatly appreciated.

Here is my response.
Since your Dad died in Missouri, one assumes his property is in Missouri. Missouri law applies to the process of distributing his Missouri property. Also, the lawyer who prepared his legal documents is not necessarily obligated to close things down at his death.

1. His Will has NOT been filed with the Court, because it is the original will (the one you have) that must be filed, by delivering it to the Court, together with a form Application for Probate of Will, a copy of his death certificate, and a filing fee. The judge will review the Will and issue an Order basically just saying it’s a valid Will and its terms will control the probate process (if probate is necessary).
2. If you are correct about the size of your Dad's estate, and probate is necessary, another form "Application for Probate and to appoint a Personal Representative" along with a larger filing fee, can be filed at the same time. The person named in the Will as Personal Representative is the person the Court presumes will be appointed. But, on occasion, for example, other family members, or a creditor, could challenge the terms in the Will.
3. If and when the Personal Representative is appointed, she (you) will have 30 days to inventory the property owned by your Dad at the time of his death, which would be the “property of the estate.”

All of this sounds complicated, and it can be. Or it can be VERY easy and quick. It depends on the nature of what’s in the estate, who the heirs and creditors are and what disputes might develop. Lawyers don't automatically get involved in probate of an estate with an out of state Personal Representative.

If a lawyer puts her name on a court file (just signing one of the Applications might be enough), she cannot withdraw from the case without the Judge’s permission. And most lawyers wouldn’t put their names on Court files without knowing what’s in the estate, (lists, values, etc.) and receiving a retainer in an amount that would cover the initial activity.