If I get 0.5ml in my lips, will anyone notice?

If you are wanting a very natural looking lip, I would recommend Volbella which is a new filler from Juvéderm that is very subtle. It's a really nice place to start. Schedule a consultation with a board certified plastic surgeon or cosmetic dermatologist to discuss. 

Hi a_k_123, thank you very much for your question. Dermal filler can be injected into and along the lips to increase fullness and refine proportions. The product and quantity used affect the outcome, but I find that the biggest factor is the physician performing the treatment. It is certainly possible to reach a subtle result that you will enjoy but will not be obvious to others. In general, fuller lips can accommodate more product subtly than thinner lips. You can always start with 0.5mL and return for another syringe if you'd like to see a more significant change. If you are concerned about maintaining your privacy, be advised that lips are especially prone to swelling and bruising. I advise my patients to schedule their appointments so they don't have significant social or professional engagements for the next few days. 
I recommend seeking out a board-certified dermatologist who specializes in injectables aesthetic treatment and shares your preference for natural-looking results. Your aesthetic dermatologist should listen carefully to your goals, provide you with a detailed explanation of the treatment plan and make you feel comfortable.

If you want to change the volume in your lips subtly without attracting too much attention, I’d suggest starting with a full syringe of Volbella. Volbella has a lovely, thin consistency that allows subtle improvements. Be warned, however, that the injections will make your lips a little sore, bruised and swollen for a few days, so if you live with family members, they may notice the difference. If not, you can start with a syringe of Volbella, try it out, and if you want more, you can add it later. Consulting a board-certified dermatologist will help you come up with a plan to get the lovely lips you want without attracting too much attention.