This is only one misconception that we tend to get from Hollywood. In Minnesota, a legal separation is not a prerequisite for a divorce. A legal separation is an entirely separate legal proceeding.
Whereas a dissolution of marriage (divorce) terminates the marital relationship between the parties, a legal separation does not. Parties to a legal separation are still married (so they can not marry anyone else), but a court order will determine the rights and responsibilities of the parties during the legal separation. Check out Minnesota Statutes for more information.
Legal separations are very rare, most likely only done for special religious or personal reasons. The process for a legal separation is very similar to the divorce process, both starting with a petition and ending with a final court order. If one spouse wants a divorce instead of a legal separation, a divorce will be granted. If the spouses later decide to divorce, they will have to go back to court and bring a divorce action. Because of this, getting a legal separation may just double the cost of the ultimate divorce.
The Minnesota Courts Self Help Center provides a good explanation of legal separations here.