How to achieve a clay render look?

Well, if everything including your walls and lights is dynamic, this technique will not really help (except may be for doing small fixes on a ‘prefab’ here or there. You will need to go with ‘real time’ shadowing or use one of the advanced techniques explained above or combine techniques (good luck with this one, I don’t think I will be able to help you there… this is just too advanced for me;)