What Neptune does in a chart
Neptune is the part of the chart that answers one question:Where do I dissolve the boundaries? It describes imagination and idealism — not events, and not outcomes.
What Leo does to it
Leo is a fire sign, so it moves on impulse and enthusiasm. It is also fixed, which means it holds things. Put those together with Neptune and you get dissolves into leo themes — warm, expressive, proud, applied specifically to imagination and idealism.
Leo is ruled by Sun, so this placement tends to be coloured by core identity as well.
The useful version
Placements are not verdicts. This one describes a default setting, not a limit — the things listed above are what you do when you are not thinking about it, which is exactly why noticing them is worth something.