Voice over

Voiceovers are a popular story telling strategy that require a bit of planning.

You will want to have full liberty throughout your shoot and post-production, but you will also need to keep track of your original vision.
Step 1: Record a Guide
If there is going to be a lot of voiceover in your movie, you should consider having the actor come in and do a casual recording of all of it before the shoot. This will be useful so that you can get the timing of your scenes right. If you have a situation where your character needs to slam the door at a specific word during the voiceover, having the recording will ensure that the two will match up in editing. These two layers of existence may seem to be separate, but your film will need to bring them together seamlessly.
Step 2: Shooting

If you chose not to record a guide, then you will still need to somehow make sure that the timing does not fall apart. So, you should have your script supervisor read the voiceover as you shoot the scene. This will not have all the inflections and nuances that the actor would have used, but will give a rough estimation. If you need to record sound during the take, then do a few rehearsals this way and time the voiceover. Let the actor get used to the timing, then do the take with the script supervisor keeping track silently. If you did in fact record a guide, then these methods will still apply only with the recording rather than a distracted script supervisor.
Step 3: Temp Track
If you chose to skip step 1, you should now record a guide for your editor. Though you did not have much of a guide during production, having a voiceover recording will make the whole editing process easier. So, take your actor and your sound recordist to a quiet corner at the end of the day. Have them record the voiceover. This will not be the final product, but you might as well make it as close to accurate and clean as possible. Now your editor has something to work with.
Step 4: Official Version
Your editor has now finished editing your film and everyone is satisfied with the cut. You now need to record the voiceover in a sound studio. This will get the most pristine sound you have ever heard. And now you can play the movie in front of the actor in order to match the new recording to the pacing of the film, rather than the other way around. Perhaps your edit has convinced you to drop a few lines. If you had recorded the voiceover any earlier, you would have been bound by its rules.
Voiceovers are great for storytelling. But, you will need to record them properly in order to get the full effect from them.