A couple of minifigure series ago, there was a British spy that had a suitcase and was holding a copy of The Financial Times. I thought it would be fun to change the ending of the famous book War of the Worlds. Where instead of the aliens dying from bacteria, they are slaughtered by the British spy found in the minifigure series with his gun that folds out of his suitcase.
Created the legs first before I thought about making an alien. I did not know what to use the legs for, until it dawned on me that I could make a scene from War of the Worlds and the legs could be the alien. The rest of the build fell into place. When I build, I normally create some obscure part of the build first and then create the whole build around it.