If you want to create an ad as wonderfully sentimental and magical as this Morrison's tv spot you need to do three things:
1. Embrace your emotions
Reject functional and rational messaging, and embrace the right brained world of emotion. Force yourself to see the world from the viewers perspective; their humanity, aspirations, frailties, and joyful moments. Resist the urge to make it mainstream or superficial (and write that in your brief).
2. Remember it's not about you, its about me!
Make the viewer (customer/audience) your only priority. That's right, tell a story only they will care about. Realise that capturing their mind and their heart is all you need to do. This is not the time for selling, for price promotions, for store front shots or gratuitous branding.
3. Believe, be fearless and stay the course
For a story this magical, you have to trust your agency, the creatives and the director. When they talk about making it feel authentic and spontaneous...just let them do their thing. When they ask for the 'slow mo' camera for the flour scene just say yes. When you ask to make the logo bigger in the end frame, listen and agree when they say no and push back.
One last thing
Whatever you do, don't give in to internal pressure to make changes (stay strong in that tough meeting when the ad is being torn to pieces). Don't cave to demands to make it more like competitor 'X', or add in more facts like competitor 'Y'. Only marketers and agencies with true belief in their art will take calculated risks like this, and customers will reward them with their minds, hearts and money.