Okay, so I have a character who calls her friend earlier in the day asking her to pick her up in the middle of the night.

The problem is that I want it to be a voice over while my MC is packing right before she leaves. How should I format this?

I'd just write it as regular voice over interspersed with the narrative-scene description of the packing, with indication of when:

MARY (V.0.)
Hi Jane! listen, I need a favor...

INT. BEDROOM - NIGHT

Mary packs her suitcase as we hear her conversation earlier in the day.

MARY (V.O.) (cont)
....I can't stick around any longer,
and I wonder could you come by tonight?
Give me a lift?

Mary shuts the suitcase and looks around the room once more. Then she turns and
leaves.

Or if you want to see the action of your MC making the call, you could do it as an INTERCUT -- making sure to include in the slug lines that the Call-action was EARLIER THAT DAY and the packing action is THAT NIGHT
(or similar)

Thanks this was very helpful
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Frenzy 08: who knows what will happen?