Do you like the photos she’s taken previously?
Look through her portfolio and find out if this is the style you want for your project to be told. Look if the images are consistent also in terms of editing so you know if you can expect the same.
Define what you want: Light and airy photos? Or dark and moody? All natural light or cool effects?
You have to decide and go with a photographer that offers this, and don’t expect that you can tell her what you prefer later on. Ask to see photos from a variety of lighting situations to make sure your photographer can handle it if you are in doubt.

Do you like the photographer?
Before booking you should definitely meet with the photographer - either on phone, Skype or in person. You have to get along. After all you will be spending the whole project together, and if you can’t trust her, then you probably won’t like the photos in the end either.
Get to know her; learn about her motivation, her experience level and what strives her.

How’s her storytelling ability?
Is she telling a story through her photos or simply taking pretty pictures? Is there details and overview photos? Pay attention to what the photographer is offering before making a choice; Just because someone is a good photographer doesn’t automatically make them a good storyteller!

What’s your budget?
Can you afford her, or switch some things around in your budget to accommodate the price of the photographer you dream of? You likely won’t be sorry you did.
Don't forget: After all it’s the photos you have that are left to tell your story and the first thing people get to see.
If they are lower in price than other photographers you should definitely find out why. Maybe she is new and wanting to add to her portfolio. Maybe she is not including as much as others you are researching in her price - so do your research to find out why.

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I hope this little guide helps you through the process of finding the right photographer for your project/your campaign. I know it can be hard - for the photographer as well! In the end just go with your gut feeling. Most likely it won't disappoint you if you considered all the steps.
Please don’t hesitate to ask any questions!

And YES - it’s a she on purpose.