I noticed the fuller-lip effect on camera long before I knew any backstage tricks, and once I dug into it I realized it's a mix of simple makeup, lighting, and a touch of visual
trickery. For starters, makeup artists are magicians: overlining with a slightly darker lip liner, filling with a mid-tone lipstick, and finishing with a high-shine gloss catches light and makes lips read larger on screen. A matte, flat color rarely gives the same plump impression.
On top of that, studio lighting and camera choices are huge players. Soft, frontal lights reduce tiny shadow lines around the mouth that normally give definition and, paradoxically, make lips look fuller by removing depth cues. High-definition cameras also emphasize detail, so that gloss and texture really pop. And yes, there are non-permanent plumping products and temporary swelling agents that can be used right before a shoot to boost volume.
From my experience watching sets and reading interviews, the public often jumps to surgical explanations, but many on-camera transformations are just clever styling, a bit of product, and the right lens — simple, fast, and reversible, which I kind of prefer.