Cover letters, unlike resumes, give you the opportunity to show a little bit of your personality as you try to make a (concise!) case that you’re a good fit for the job. In addition to highlighting a few key points that prove your skills align with the requested qualifications, you can take a sentence or two to explain why you’re interested in the job. Hiring managers want employees that are excited about the company or position, and a brief sentence indicating that there’s a personal reason you're applying for the job will go a long way.
But be careful not to go overboard, as you risk coming across as inauthentic or downright creepy. Keep it short -- it’s great to say you’re “passionate about scripted comedy” or “a long-time watcher of ABC's TGIT block.” If your fandom goes deeper -- let’s say you blog about every detail of the cast members’ personal lives or spend your weekends cosplaying as Olivia Pope -- leave it out of your cover letter!
You're entitled to have whatever strange hobbies you choose, but if you include them in your cover letters, you’ll come across as a complete weirdo, because there's not enough context for the employer to know that you're not some maniac Kerry Washington stalker. Save the details of your passion for your interview, when you have the opportunity to express yourself face-to-face and share your story in a less overwhelming way. As long as you’re not staring your interviewers down like a serial killer while you tell your story (if this is a problem for you, please take advantage of our mock interview services!), they’ll see your superfandom as an interesting and unique part of your personality and will understand how much this job means to you. But tone it down for cover letters -- it doesn’t take much to prove you’re enthusiastic, and you don’t want to risk sounding like a creep.