While the voice over is being delivered, visuals play. To represent this, the description of the visuals - the Action - ought to be commenced before the voice over is presented in the script
Penguins frolic on the ice. MORGAN FREEMAN (V.O.) Penguins are cute and sometimes they fall over. A particularly cute chick goes head first into a rock.
If there a script contains a lengthy voice over, it is often wise to have the VO speech alternating with action, as it gives a better idea of the continuity.
If your VO segments are too long, try giving your character more things to do so it keeps the visual interesting. If the character isn't doing much, use lots of cuts and changes of scenery.
Voice overs, along with flashbacks and parentheticals, are commonly overused narrative devices. Consider alternative (visual) mechanisms for delivering the information you want to impart. For example, feelings and emotions are most of the time better portrayed by actors than by their overheard inner voices.