Theo and Diego-san have a complicated relationship. Where Theo and Thompson can fight and wrestle one minute and sit there cleaning each other and sleeping next to each other the next minute, the usual interchange between Theo and Diego involves Theo entering a room and Diego then leaving. Theo chases Diego around but that usually ends with Diego hopping up somewhere high enough where Theo can't reach him.
There was one evening when Theo and I were playing fetch and after about a zillion throws I decided I needed to get some other work done. Theo dropped the ball at my feet and looked up. I refused to look back. Eventually he picked up his ball and brought it over to where Diego-san was lounging and dropped it at Diego's feet. He gave Diego the same look as he was giving me, hoping I guess, that Diego would play fetch with him. Wasn't going to happen.