Consider the following example: A resume comes in that lists two years of work experience as a realtor, one in a medical office, and an internship at a record label before that. The cover letter lists some generic strengths and, actually, if the hiring manager was looking for a realtor, she’d have found her guy. Only she’s not looking for a realtor, she’s looking for a coordinator in the distribution department of a major studio. And she’s also not a mind reader. So the hiring manager has no idea that the job applicant has always been interested in entertainment but needed a more stable job after college for whatever reason. Since she's hoping to hire someone who's passionate about entertainment, she's already questioning whether she should bother to bring the candidate in for an interview. Don't let that happen to you. All it takes is one quick sentence to explain your intentions in a cover letter, so make sure your goals come across clearly.
But the cover letter isn't our candidate's only problem -- his resume bullet points don't match the qualifications listed in the job posting. The hiring manager doesn’t have the time to assess what skills a realtor might have that could benefit her company, so his resume goes straight into the trash. The realtor made the mistake of basing his bullet points on the most obvious aspects of his previous position instead of refocusing his resume to target the job he wanted. He should have thought more about the skills he mastered as a realtor that are applicable to a coordinator position. “Managed a database of properties across the Los Angeles area and generated analyses and reports to present to upper management for company development” aligns with “Manage database of theatrical exhibitors; run reports and analyze ticket sales across demographics” a whole lot better than “Sold houses.” Don't force the recruiter to read between the lines -- spell it out for her.
To recap: If you’re transitioning across sides of the industry, back into entertainment, or out of an unrelated field, explain your interests in your cover letter and make connections between your previous job responsibilities and what the posting is asking for. Your resume is a sales tool, and the recruiter is your customer. Target your messaging, and convince her that you can handle a career transition.