If you're interviewing for an assistant position, you'll likely get asked about your long term goals. Maybe you want to be a screenwriter, but you know that starting off at an agency could help you build the connections that will lead to a writer's assistant job. Now you're in a bind: Do you give the most transparent answer and lose the job to someone who's in for life, or do you spin the truth?
If you want the job, you should definitely choose the latter. Say something like: "I've typically been partial to writing, but I'm really interested in X about representation and could see myself in that role as well. I'm open to whatever the future might hold and will be completely committed to the position and to supporting my boss." The X should be something true, and, if you think about it, you haven't technically lied -- you never know when you might change your mind.
But be careful: when you're applying for internships, you don't want to use this spin. In fact, you want to be as clear about your future goals as possible, so your supervisor will know what your expectations are and what to teach you when you get the job.