I'm interested in someone, but the only time I see them

go for it...without appearing too aggressive

Just don't draw attention to it. If you need to start being more conversational with them during the customer interactions. Some people think they can't talk to employees but service is a number 1 priority and everyone gets joy out of it, and it increases relations with customers.

Ask if they'd like to get a coffee or some generic bullshit. Don't ask when they're shift ends, it comes across a little weirder then youd think.