For what software? The software you are working in and it's capabilities will greatly effect how to go about this. Also, there really is no way to do any of this without detail and quality loss, the trick is doing it in ways to minimize the visible resulting drops in quality and detail. There will always be a trade off, it is the nature of the beast.
As to doing these things in real time, this sounds more like a game engine question than a 3D render question. Whatever software you use will determine any real tine options, and this is going to depend heavily on how good is your graphics card in almost all cases. Most of the software focused in this forum (Daz Studio, Poser, Carrara) do not have real time options like this. While I know Lightwave, and at least Cinema 4D do, it is based in certain plugin modules and how good is your graphics card.
To cover everything you are asking would take pages and pages. First there would need to be a common point of reference. My best suggestion would be to do targeted searches on Youtube for tutorials in the software you are using for each of these topics, Ultimately to optimize a scene you will want to do a variety of LOD, Culling, Optimizing Meshes, Optimizing and reducing textures where possible, replacing resourcve intense distant figures with low poly proxies or even billboards. If your software supports Normal Passes, render to that and use an intense video package to apply lighting (Some great information on this tied in to the production process of Arcane).