I'm both a young writer and currently working my first job in publishing at a Big Five publishing company. I'm not sure where I'm planning to go next career-wise, but I have considered trying to move over to the agenting side of the business, and I did previously intern at a literary agency and interview for several internships and assistant positions at different agencies. My question is, how do I handle querying an agency that I previously interviewed with, and how do I handle applying for a job at an agency that once requested my full manuscript?
You keep quiet about the writing. Don't mention you've queried, don't mention you write. If you're applying for a job, focus on the job they want you to do. Don't tell anyone you're really just here till you sell your novel.
I cannot urge you strongly enough to NOT consider working at lit agency while trying to forge a career as a writer.
It takes a lot of time and attention to become good at a job. Literary agenting isn't 9-5 or even 9-9 if you want to be good at it. You're thinking about work even when you're out at brunch with your pals.
And if you're an assistant, it's worse.
The problem is: writing is the EXACT same kind of work. It's not 9-5 even if you only write for a couple hours a day. Writing a novel of any kind requires your creative brain to engage, and there's no schedule for that.
If you want to be a writer, you want a job you don't take home with you after quitting time.
The other part of the problem is wearing two hats in the publishing industry at this stage of your career means it's almost impossible to network effectively. You've got to build your career as one thing, then branch out later when you've got an established base.
If you ignore this, and apply for a lit agency job anyway: do not mention anything about writing.
I know you think you're the exception to this, that you can do everything you want and do it well. That's exactly the kind of bravado and confidence required to be a good agent, BUT you also need to be realistic. It's very very hard to do two things very well (just ask Michael Jordan.) Do you want to be really good at one thing, or sort of ok at two? That's the real choice here.