There are varying levels of dementia; ranging from a person who may simply act confused all the way to someone who's near comatose.
My Uncle was, thankfully, far closer to the latter category. He would repeat himself a great deal, lose track of what he was doing, and become quite stubborn. During this time, he showed the greatest level of comfort when he was in familiar places. We did what we could, even tracking down an identical 80's model TV when his own broke.
At his worst, he became quite angry, as everything was confusing to him. It was easier to let him talk than to talk back, because he couldn't keep up. At times it was obvious that he had no idea who you were, and any number of times he referred to me by my dad's name. It just became a game of keeping my chill.