This was Kurosawa’s last film with his own protégé Mifune, star of Rashamon and Seven Samurai. One rumour as to why is that ‘Mifune's natural beard had to be maintained through the lengthy production, so he was unable to act in other films’ causing financial stress.
There was an interesting documentary on him, which I’m unable to breakdown, so I copied the following from Wikipedia:
He was known as "Tenno", literally "Emperor", for his dictatorial directing style. He was a perfectionist who spent enormous amounts of time and effort to achieve the desired visual effects. In Rashomon, he dyed the rain water black with calligraphy ink in order to achieve the effect of heavy rain, and ended up using up the entire local water supply of the location area in creating the rainstorm. In the final scene of Throne of Blood, in which Mifune is shot by arrows, Kurosawa used real arrows shot by expert archers from a short range, landing within centimetres of Mifune's body. In Ran, an entire castle set was constructed on the slopes of Mt. Fuji only to be burned to the ground in a climactic scene.