If it is a Captain who is named acting Boss,then I would imagine that he would name a crew member to move up to acting Captain to look after his crew during his tenure. If the Acting Boss becomes Boss,then the Acting Captain would probably be made Captain.

If an Underboss or Consiglieri becomes acting Boss,he could probably handle the added responsibility without having to put someone in charge of his operations.Generally,these positions are filled by men who are close to the Boss,know how he thinks,and have been in on higher level decisions.
Also they are not burdened with day-to-day operations like their underlings.

In either case,an Acting Boss is rarely in danger of making a bad call,because usually,he is filling in for an imprisoned Boss who will no doubt make his wishes known during visits from his "lawyers". If it becomes apparent that the Boss isn't coming back anytime soon,then the Acting Boss will probably take the top seat.

Of course with these goons you never know. I believe that Carmine Persico is still the official Boss of the Colombo Family even though he went in after the "Commission Trial" and will never be on the streets again.

Gotta wonder how in the hell that's working out.