Suppliers are friends... not food.

Raket Rule number 15 (or was it 182?) --Keep Connected with the right people.
Suppliers can make or break that project you've worked on for weeks, even months.  This is why a good rule of thumb is to keep a good slew of contacts from the production world.  Oh wait, not contacts.  Friends.  Uh-huh.  A good relationship must be happening between you two.  Not just a singular project in the past.  But a more meaningful connection.  We're talking about massive trust here.
Why so?  Example!

Just because a photographer can shoot pretty still pictures of cooking oil and food preps does it mean that he can shoot a neat ad of a kid and his dog.  Not happening.  Unless you know the photographer to be gifted with patience and people skills.  Or even at least a hint of proper motivation and venue sense.  That's why you have to know the guy beyond his portfolio.  Because if he can't stand an energetic 7-year old and can't deal with a mini pinscher (one of the liveliest dog breeds on the planet) --then there's just no hope.
You can tell I'm speaking from a bad experience here, can't you?

Ok, ok.  This also applies to that production house that's been making absolutely fab quick cut and paste AVPs for you without fail.  If you have to get them for a pesky client who wants his TVC as visually epic as The Apocalypse, then you should know if your team is in fact movie savvy.  Through and True.

A recording studio that has given you the best jingles can't always deliver a radio drama anthology either.  Unless your suppliers are rabid researchers and thrive on new experience immersion.  THAT you'll have to find out.  And that's why you have to know your suppliers on a personal plane, not just because they produce pretty sounds, pretty pictures.

More than partnership, it's friendship.  That's how you produce great work.  Between friends you can trust with your all.