The #1 quick-fix to indicate “I’m in front!” is overlap. Other depth cues on a flat surface include: diminution (getting smaller-ness), convergence and upward position on field. These 2D figures at San Francisco's Transbay Temporary Terminal keep a polite distance and staying within the boundary. For their work-weary audience, they model all the depth cues but overlap, in the hope that the audience for this sign will behave, in real space, accordingly. The figures don’t overlap each other; that might imply touching! How telling for our culture! The only overlap in this pic is that of shoulders covering the farther line boundary.